Anything you do to burn calories (i.e. any exercise) will make you lose fat, unless you eat a lot to regain those calories. But if you keep a healthy diet and then bike, you will lose fat, not just in your legs but all over your body. But your legs will get more muscular, as well as your upper body and core (biking works out more than just your legs!)
The good news is that the more muscle you have, the more calories your body will burn when you exercise, so the more you bike, the faster you'll start to lose fat.
