A framer is akind of constructor or builder; a cowboy is a kind of farmer (in the widest sense) and the letters can construct the other word "framer". Seems sensible enough to me. Whereas a prefab has nothing particularly to do with cowboys, surely. But we don't even know what letters are already "given".
Without the final "b" & without knowing this is a themed but not cryptic puzzle, my suggestion still seems perfectly OK to me.
Ducksie: maybe your q & criticism crossed with my explanation.
And to the original poster: maybe you could give any letters you have automatically next time --- or didn't you have any?
This was all rather a waste of time under those conditions, I must say!