Sully; slander (5)
I believe the answer is:
smear
This clue is a double definition.
'sully' is the first definition.
(I know that sully can be written as smear)
'slander' is the second definition.
(I know that smear is a type of slander)
(Other definitions for smear that I've seen before include "Slanderous denigration of character" , "False report" , "Dirty mark; aspersion" , "Greasy or sticky mark" , "Unwarranted accusation" .)