Pollute a narrow pass (6)
I believe the answer is:
defile
I believe this clue is a double definition.
'pollute' is the first definition.
(I've seen this before)
'a narrow pass' is the second definition.
(I've seen this before)
(Other definitions for defile that I've seen before include "Foul - pass" , "dishonour?" , "Pollute, make dirty" , "Spoil; narrow gorge" , "Pollute in ravine?" .)