Spoil or impair (3)
I believe the answer is:
mar
This clue is a double definition.
'spoil' is the first definition.
(I know that spoil can be written as mar)
'impair' is the second definition.
(I know that impair can be written as mar)
(Other definitions for mar that I've seen before include "Spoil or deface" , "when spring growth starts?" , "Make imperfect" , "Ram into and spoil" , "Detract from" .)