If you're exercising and not over or under eating, you're going to lose fat. Unfortunately, there really is no way to "spot" lose problem areas. The good news is, you're not just going to lose "leg fat" by cycling either. It will be more of an all-over body change. It is a great exercise and very challenging. I've gone once and thought I was going to barf! Huge amounts of calories can be burned in a one hour class!
So to answer your question, yes, spin class will help you lose your belly fat, as well as the fat in the rest of your body, all in proportion.