Besmirch quisling in retreat (3)
I believe the answer is:
tar
'besmirch' is the definition.
(I've seen this in another clue)
'quisling in retreat' is the wordplay.
'quisling' becomes 'rat' (I've seen this in another clue).
'in retreat' says the letters should be written in reverse.
'rat' backwards is 'TAR'.
(Other definitions for tar that I've seen before include "Naval man" , "Sticky stuff for a kind of sailor" , "Black substance, used on roads" , "Informal sailor or road-surfacing material" , "Dark viscous flammable substance" .)