Foul pass (6)
I believe the answer is:
defile
I believe this is a double definition.
'foul' is the first definition.
(I know that foul can be written as defile)
'pass' is the second definition.
(I know that defile is a type of pass)
(Other definitions for defile that I've seen before include "Make dirty in narrow gorge" , "Spoil; narrow gorge" , "Desecrate" , "Corrupt - mountain pass" , "dishonour?" .)