As long as the lines of the lineart are vector paths, they can be used as a clipping path.
I'm not sure if paths which have any LPEs applied would work, but probably. I'll test.... Well no, that doesn't seem to work. But the solution should be fairly simple.
Duplicate the path that you want to use as a clipping path, and remove the path effects. Once it's a regular path, it should work as a clipping path.
Since we don't have access to the SVG file, or know enough about the file, I'm not sure about this. But it sounds like it might be possible to avoid having to use clipping....assuming it's just one color. (And it's always better to use as little fancy stuff as possible, if you have a choice.) If you applied your color using the Paint Bucket tool, and it overlaps the lineart, maybe you don't have to use clipping, to make it fit.
After you have combined all your paths together (the ones you want to use for a clipping path) and it's now a single closed path, you can just use a regular fill. Select the path, and either click a color from the palette, or create one using Fill and Stroke dialog (or choose one from an imported raster image, using Dropper tool).
However, if the colors that you want to clip are more than 1 color, clipping might end up being the best solution.
Let us know how it goes
