Copying framebuffer to texture - with alpha

I have a number of complex sprites made up of very simple parts that don't change that often, and redrawing the parts real-time is a pretty big waste of cycles. So i'm trying to push the entire sprite onto a texture, and just use that texture as my rendering call for the sprite.

This is all great, but whenever i try to copy, the texture i get is an image with the sprite on a black background.

This makes sense, i'm clearing the buffer and drawing the entity, then glCopyTexImage2D() straight to the texture. Naturally i'll get whatever's in the gaps, which in this case is my clear color, black.

I obviously can't have great black squares following my sprites, but i also can't get them into their texture without rendering them somewhere first. Is there a way to render directly to a texture, or to specify that i want the untouched portions of the texture to have zeroed alpha?


Note; i tried a real hack that involved clearing to an ugly purple (1, 0, 1, 1) and running a 'replacement' shader that ignored fragments that would be purple whenever rendering the fragments of the sprite's texture, but this resulted in a sort of purple 'border' around everything, and the shader didn't quite work correctly anyway.