Spoils instrument in retrospect (4)
I believe the answer is:
loot
'spoils' is the definition.
(both can mean a reward or proceeds)
'instrument in retrospect' is the wordplay.
'instrument' becomes 'tool' (synonyms).
'in retrospect' says the letters should be written backwards.
'tool' written backwards gives 'LOOT'.
(Other definitions for loot that I've seen before include "Plunder, pillage" , "Booty, spoils" , "Tool found among plunder" , "Plunder - booty" , "Steal property (in war)" .)