Taint, besmirch (5)
I believe the answer is:
sully
I believe this clue is a double definition.
'taint' is the first definition.
(I know that taint can be written as sully)
'besmirch' is the second definition.
(I know that besmirch can be written as sully)
(Other definitions for sully that I've seen before include "Soil or tarnish" , "Make dirty" , "Taint" , "Stain or tarnish" , "Besmirch" .)