Note that employers don't like to hire overqualified people because they won't stay around long. But since it is probably obvious that you're overqualified, admit that you are, but also emphasis the positive.
For example, "I am overqualified in some ways. I have more experience that is required for this job, but you are looking for someone who is an expert in X, and that's me. However, that doesn't mean I'm completely overqualified. I feel that I have much to learn in the area of X, which is a big part of this job and I know it will keep me challenged . . . ."