I would agree that "eating clean" could be only eating as naturally as possible, for some people. The human body has evolved to eat natural and relatively unprocessed foods and it therefore makes sense to eat in this way. BUT I do think that there is further criteria too.
I do think that "eating clean" greatly depends upon the context in which the person is using it. A person who just wants to eat in a healthy way would fall under the above definition.
However a bodybuilder's definition may be vastly different. Clean eating may also require supplements in order to aid the muscle building process (protein shakes, L-Glutamine, dextrose, creatine, vitamins/minerals etc.) - all of which are highly processed. In addition to this, processed white rice is often a staple of many bodybuilders.
Same deal with most athletes, in order to follow a clean diet, many athletes consume supplements alongside alternative diets in order to reach a level of peak performance.
So personally I would define "eating clean" as consuming the most appropriate diet that supports that particular person's goals. These goals may be anything - building muscle mass, losing fat mass, general health & wellbeing, improving strength, increasing endurance capacity and so on.