Senior officer in a lockup (3)

I believe the answer is:

col

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'senior officer' is the definition.
(abbreviation for colonel)

I don't understand how the remainder of the clue works.

(Other definitions for col that I've seen before include "Somewhere up on the mountain" , "Mountain defile" , "High mountain pass" , "Low area between peaks" , "Mountain saddle" .)

I've seen this clue in the Evening Standard.

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