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