Damage, impair (6)
I believe this is a double definition.
'damage' is the first definition.
(I know this)
'impair' is the second definition.
(I know this)
(Other definitions for deface that I've seen before include "ruin" , "Spoil or damage the surface of" , "Deliberately spoil the appearance of something" , "Spoil surface of, mar" , "blemish" .)
This clue is a double definition.
'damage' is the first definition.
(to mar is to spoil or damage)
'impair' is the second definition.
(I know that impair can be written as mar)
(Other definitions for mar that I've seen before include "wreck" , "'Impair, spoil (3)'" , "hurt" , "A windy month" , "Oh dear!" .)