What does that even mean ? Well, it’s quite simple. It means ditching food that has additives eg preservatives, thickeners, colourants, stabilisers and chemical antioxidants, for starters. It means not eating added sugars (however labelled). Added starches too, like rice flour and potato starch.
Sometimes, it means delving into the internet to ascertain whether lactic acid, ascorbic acid, citric acid and substances like tocopherol and carrigeenan are healthy or not.
There are people, who will call milk, butter, cream, kefir, yogurt and cheese processed food. Obviously, not all of these products are produced by factories. But the method of production matters for many people, even if they don’t eat organic.
Clean eating has been criticised. Some people say that it implies that other food is somehow ‘dirty’ and substandard.

In the USA, a study released in July 2019, analysed 230,156 foods and found that 71% of products such as bread, salad dressings, snack foods, sweets and sugary drinks, could be considered ultra-processed. 86% of products from the top 25 manufacturers, could be considered ultra-high processed.
More than 70% of America’s packaged food is ultra-processed — and it’s a big problem (zmescience.com)
For every 10 calories eaten in the US, 8 come from store-bought food, both packaged and unpackaged.
The prevalence of, and acceptance of, processed food is such, that many American cookbooks will include mayonnaise, ready-made frosting, soup mixes, soups and jello (jelly) in ways which British cooks just wouldn’t do. We don’t reach for a packet soup mix, to make a dip. We don’t often add mayonnaise to a cake. We don’t drown Iceberg lettuce in lime green jelly, for sliceable salad.
American food doesn’t have much labelling and unless certified, a claim doesn’t mean a whole lot. So yes, food standards in the USA are worse than in the EU. More people suffer from food poisoning and more people get chronically sick, from the food they eat. American food is very often, substandard.
Maybe that’s why Americans eat more organic food than any other nation ? In Britain, organic food is often seen as being for the wealthy. In the USA, budget supermarkets such as Aldi, Lidl, Target, Trader Joe’s and Walmart have own-brand organic ranges.

Britain is not a paragon of virtue. Brits are the fattest people in Europe. For many people, food is fuel, rather than a pleasure.
In mainland Europe, before the pandemic, lots of people did stop for lunch and go to local restaurants or go home. In Britain, the norm is a packed lunch or some fast food, often eaten at your desk. . It does seem that Britain looks to the USA rather than mainland Europe, when it comes to food culture.

Manufacturers are always looking to add cheaper ingredients, to make more profit on their product. They call it Added Value, but it’s for them, not us. Manufacturers of organic food are no exception. Organic agave syrup in pickled beetroot and organic glucose fructose syrup in pesto, for example.
GRAS means Generally Regarded As Safe. It means that the food additive won’t kill you instantly. It doesn’t mean that it’s safe, in conjunction with other chemicals, over your lifetime.

Critics say that Clean Eating discriminates against poorer people. That may be true, if you live in the USA, in a neighbourhood with poor public transport and don’t have a way of getting to supermarkets. Bodegas may not have much/any fruit and veg.
In Britain, most communities will have local shops. When new housng estates are built in the country, there are planning rules, which mean that the developers have to extend an existing primary school or build a new one, build a health centre or a small mini-market or a few shops, depending on the stocking density, as it were.

With Brexit, Britain will be under pressure to lower standards, to meet American demands. Most British supermarkets say that they won’t stock chlorinated chicken, for example. Looking on labels will become even more important.
It is not always more expensive, to buy foods without additives. Also, from January 1st 2021, food from EU member states will still retain high standards, whereas British food manufacturers may choose to import fewer ingredients from EU member states and switch to processed food or ingredients from USA, China, Brazil, Vietnam and Thailand, for example.

Not all processed food is unhealthy. Live yogurt, for example, has health benefits. You might prefer your fish to be wild-caught rather than farmed, but your budget might not stretch to it. A vegetarian or vegan diet can be healthy, if you make everything yourself. If you eat veggie burgers, garden burgers, vegetarian sausages and pies at every meal, you will ingest industrial seed oils, diglycerides of fatty acids, palm oil, partially-hydrogenated fats, preservatives etc. No more healthy than eating processed food aimed at omnivores.
It’s difficult, to eat clean in a world with unclean food. If avoiding the nasties in food is your aim, just do the best you can, in your situation. No one is perfect.