Commanding officer left mountain pass (3)
I believe the answer is:
col
'mountain pass' is the definition.
(col is a kind of mountain pass)
'commanding officer left' is the wordplay.
'commanding officer' becomes 'co'.
'left' becomes 'l' (common abbreviation).
'co'+'l'='COL'
(Other definitions for col that I've seen before include "Depression among ecologists" , "Feature of mountain" , "Pass between mountain ridges" , "Low point of a ridge" , "Depression in range of mountains" .)