Spoil, tarnish (3)
I believe the answer is:
mar
I believe this is a double definition.
'spoil' is the first definition.
(synonyms)
'tarnish' is the second definition.
(I've seen this in another clue)
(Other definitions for mar that I've seen before include "Spoil something" , "Make imperfect" , "Ram into and spoil" , "Spoil, damage" , "A windy month" .)