Fellow traveller maybe three miles south of mountain pass (9)
I believe the answer is:
colleague
'fellow traveller' is the definition.
(person who shares one's aims or causes)
'maybe three miles south of mountain pass' is the wordplay.
'maybe three miles' becomes 'league' (historical unit of distance of roughly 3 miles).
'south of' is a charade indicator (letters next to each other).
'mountain pass' becomes 'col' (col is a kind of mountain pass).
'league' put after 'col' is 'COLLEAGUE'.
(Other definitions for colleague that I've seen before include "fellow member of staff" , "Workmate" , "Fellow-worker" , "Person with whom one works" , "Fellow worker" .)