I couldn't think of a better description than the title - here's some detail:
I'm using inkscape to create maps for a board game, and it's working great so far. The game uses Hex maps which I can easily create using

Most of the maps are rectangular, that is, they have a rectangular shape that is then filled with the little hexagons. The hexagons tile perfectly and are cut off by the frame. So far, so good.
Some of the maps I want to create in the future have circular shape or other, more complex ones. That is, they still consist of the same hex tiles but the frame that cuts them off is circular.
The way I figure, I need to create a circular shape as the frame, which has to be on top of the Hex grid. The inside of that frame has to be transparent, as I want to see the hex grid through it. The outside, however, has to be filled in some colour so as to create the impression that the map is "cut off" by the frame.
Perhaps some of you can think of a way to make this happen; I only know how to fill the inside of a shape.