I've flown with a student pilot who had a bit of a stutter. I don't want to discourage you but there's more to communication in and around the aircraft than just talking on the radio. You'll also need to be able to keep up a running commentary of what you're doing. Your instructor may ask you questions at different point during the flight and there's not that much extra time available sometimes. Especially when you're going to be flying commercially, you'll need to be able to operate in a two-man cockpit environment, with all the communication needed for that. I cannot remember what happened to that particular student, but I do remember that his progress wasn't as smooth as for other students. In short, it may not be impossible, but you will have more of a challenge than others.