Pollute narrow passage (6)
I believe the answer is:
defile
I believe this is a double definition.
'pollute' is the first definition.
(I've seen this before)
'narrow passage' is the second definition.
(geographical term)
(Other definitions for defile that I've seen before include "Pollute in narrow gorge" , "Contaminate" , "Foul - pass" , "passage" , "dishonour?" .)