Impair, tarnish (3)
I believe the answer is:
mar
I believe this clue is a double definition.
'impair' 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 "Cause harm" , "Detract from" , "Spoil, damage" , "Spoil something" , "Ruin" .)