Spoil ram rotten (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'ram rotten' is the wordplay.
'rotten' indicates an anagram.
'ram' is an anagram of 'MAR'.
(Other definitions for mar that I've seen before include "when spring growth starts?" , "Detract from" , "Cause harm" , "A windy month" , "hurt" .)