Officer unaccompanied through mountain pass (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'unaccompanied through mountain pass' is the wordplay.
'unaccompanied' becomes 'lone' (I've seen this before).
'through' indicates putting letters inside (I've seen this in other clues).
'mountain pass' becomes 'col' (col is a kind of mountain pass).
'lone' going into 'col' is 'COLONEL'.
(Other definitions for colonel that I've seen before include "Commander" , "Army rank" , "Military rank" , "Bogey perhaps" , "'No cello for this officer, strangely (7)'" .)