Contaminates, fouls (8)
I believe the answer is:
pollutes
I believe this clue is a double definition.
'contaminates' is the first definition.
('pollute' can be a synonym of 'contaminate')
'fouls' is the second definition.
(I know this)
(Other definitions for pollutes that I've seen before include "Makes dirty, contaminates" , "Contaminates, fouls" , "Contaminates water or air" , "Contaminates, corrupts" , "Contaminates, befouls" .)