Officer, one taking line south of mountain pass (7)
I believe the answer is:
colonel
'officer' is the definition.
(colonel is a kind of officer)
'one taking line south of mountain pass' is the wordplay.
'one taking' becomes 'on' (eg one taking drugs is 'on' drugs).
'line' becomes 'el' (el is a kind of line**).
'south of' means one lot of letters go next to another.
'mountain pass' becomes 'col' (col is a kind of mountain pass).
'on'+'el'='onel'
'onel' after 'col' is 'COLONEL'.
(Other definitions for colonel that I've seen before include "Army three downn" , "Bogey perhaps" , "Comissioned military officer" , "High-ranking officer" , "Senior army officer" .)