Dent or ding (3)
I believe the answer is:
mar
This clue is a double definition.
'dent' is the first definition.
(I've seen this in another clue)
'ding' is the second definition.
(I've seen this in another clue)
(Other definitions for mar that I've seen before include "Blight" , "Disfigure" , "Make imperfect" , "'Impair, spoil (3)'" , "hurt" .)