Defile, sully (5)
I believe the answer is:
taint
This clue is a double definition.
'defile' is the first definition.
(I know that defile can be written as taint)
'sully' is the second definition.
(I know that sully can be written as taint)
(Other definitions for taint that I've seen before include "Source of infection" , "Defile" , "Adulterate" , "Blemish" , "Corrupt" .)