It is best not to replace real whole food with protein, because in the end of the day real food will always be better than just protein. It is best to do as @muskate said - reduce the amount of food you eat with each meal, but don't reduce the number of meals you eat each day. This way, you would be using a protein shake once or twice per day to make up for your protein needs that were not filled with the smaller meals. Also, if you are trying to lean out, make sure to be using Whey protein isolate, as it is the purest form of protein that does not contain carbs or fats. I would really recommend using the Isolation by N2BM - it is a pure isolate, it tastes great, and the price is very good too.