Pass for a senior officer (3)
I believe the answer is:
col
This clue is a double definition.
'pass' is the first definition.
(col is a term for a mountain pass)
'for a senior officer' is the second definition.
(abbreviation for colonel)
(Other definitions for col that I've seen before include "Pass between mountain ridges" , "Officer" , "Low point of saddle between two peaks" , "A depression in mountain range" , "Low point of a ridge" .)