Spoil butter that's brought back (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'butter that's brought back' is the wordplay.
'butter' becomes 'ram' (a ram is something which 'butts').
'that's brought back' shows that the letters should be reversed in order.
'ram' backwards is 'MAR'.
(Other definitions for mar that I've seen before include "Oh dear!" , "Blemish" , "Spoil or deface" , "Make imperfect" , "Spanish main?" .)