Pollute a narrow valley (6)
I believe the answer is:
defile
This clue is a double definition.
'pollute' is the first definition.
(I've seen this in another clue)
'a narrow valley' is the second definition.
(I've seen this in another clue)
(Other definitions for defile that I've seen before include "Spoil; steep-sided gorge" , "Create pollution" , "Befoul" , "valley" , "Corrupt - mountain pass" .)