I don't think that there is one defintion for what 'clean eating' means. Every diet or eating plan has its' own version of clean. For example, the Zone (which is what my own eating most closely resembles) says that clean eating is balancing protein/carbs/fats in a 40/30/30 ratio and that you should seek the 'best' sources in each of these categories in your daily food intake. For protein, 4 ounces of lean chicken breast is a 'best' source, 4 ounces lean ground beef is a 'good' source, and 4 ounces of hot dog is a 'poor choice'. In his book he gives ratings for tons of protein, carbohydrate, and fat choices.
For me personally I think that the closer you can get to how the food exists naturally the closer you are to eating clean. Sauteed mushrooms don't exist in nature, raw mushrooms do (although sauteed with some butter and garlic, YUMMY). And I loved the apple analogy by another poster. So the more you can eat foods in their natural state, the closer you are to 'clean'. The more the food becomes cooked, processed, or otherwise manipulated the less 'clean' it is.
Just my thoughts,
Sabine