Damage or spoil (3)
I believe the answer is:
mar
I believe this clue is a double definition.
'damage' is the first definition.
(to mar is to spoil or damage)
'spoil' is the second definition.
(synonyms)
(Other definitions for mar that I've seen before include "Spoil something" , "A windy month" , "Make imperfect" , "Ruin" , "wreck" .)