What's left over (8)
I believe the answer is:
residual
'what's left over' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for residual that I've seen before include "Leftover - used rail (anag)" , "Concerning what is left over" , "Remaining at the end" , "'Remaining, left over (8)'" , "Remaining; repeat royalty" .)