Filling a Circle

1

Hi all, I think this is a rather easy effect, but one of those things that has its trick. What I want to do is have a circle, that is being filled, like a clock, but using a mask or however it would be done, have it going filled up. Understand? I tried using shape tweens but of course it just deforms from one state to the next instead of smoothly filling up. If anyone could throw me a tip it would be a great help.