Radial reveal masking

I’ve been at this for a bit and can’t quite figure out a way to do it.
It’s a basic masking effect im going for between two photos. First you see the one photo, then the second one gets masked in on top of the first in a radial motion. Kind of like the seconds hand on a clock rotates around, the second pic is revealed. If you have iMovie you’d know what I’m talking about. Just a simple radial transistion.

Problem is when i make a movieclip to use as the mask it contains masks itself.(in order to create the circular pie graph rotating effect).

Anyone got any ideas on how to achieve this, if you even know what i’m talking about?