Sully narrow pass (6)
I believe the answer is:
defile
This clue is a double definition.
'sully' is the first definition.
(I know that sully can be written as defile)
'narrow pass' is the second definition.
(I've seen this before)
(Other definitions for defile that I've seen before include "Create pollution" , "Befoul" , "Corrupt - mountain pass" , "Corrupt or violate" , "Sully; narrow gorge" .)