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