Pizza can be "clean" -- use whole wheat crust, don't go overboard with the cheese, and choose toppings which are vegetables or lean meats. But a typical pizza with white bread crust and pepperoni and sausage toppings that leave a grease stain below is not "clean".
Generally, "clean" eating tends to include:
vegetables
whole fruit
lean meats
good fats (unsaturated fat, including fish fat)
legumes
nuts (though be careful if you are limiting calories)
whole grains
dairy products (skim or 1% fat if you are concerned about the calories or saturated fat; plain yogurt, kefir, cheese, and similar fermented products are lower in sugars)
while avoiding or limiting:
added sugar (including high fructose corn syrup and other types of sugars like honey, evaporated cane juice, etc.)
bad fats (any hydrogenated oils; if your blood LDL cholesterol levels are a concern, avoid saturated fats as well)
non-whole grains
fruit juices are marginal at best (concentrated sugar, less or no fiber)
most processed food
If you are following any kind of special diet, then there may be some foods in the above "clean" list that you may not be able to eat on the special diet.