Blot, stain (3)
I believe the answer is:
mar
I believe this is a double definition.
'blot' is the first definition.
(I've seen this in another clue)
'stain' is the second definition.
(I've seen this in another clue)
(Other definitions for mar that I've seen before include "Disfigure" , "Spoil or deface" , "Ram into and spoil" , "Spoil stuff" , "Detract from" .)