Make an object appear just 'inside' other? mask-like effect

I have a static level and a circle that appears randomly on screen. But I want that the level just be visible on that circle... I think is like a mask effect. How can I achieve this?

Thanks.

You can see the example on the template. Basicly you create a new layer with own texture, and then you have black sprite with white circle

Hehe thanks, is working fine now