Pass left behind by the commanding officer (3)
I believe the answer is:
col
'pass' is the definition.
(col is a term for a mountain pass)
'left behind by the commanding officer' is the wordplay.
I cannot quite see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is located in the answer.
'officer' could be 'co' (Commanding Officer abbreviation) and 'co' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for col that I've seen before include "Low point of saddle between two peaks" , "dip?" , "A pass through ridge" , "High mountain pass" , "Mountain defile" .)