Backing spoils instrument (4)
I believe the answer is:
tool
'instrument' is the definition.
(I know that instrument can be written as tool)
'backing spoils' is the wordplay.
'backing' is a reversal indicator.
'spoils' becomes 'loot' (both can mean a reward or proceeds).
'loot' written backwards gives 'TOOL'.
(Other definitions for tool that I've seen before include "For example, saw" , "Drive motor skilfully" , "Gun" , "Utensil; implement" , "Working instrument" .)