Army officer in the Colditz story (3)

I believe the answer is:

col

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'army officer' is the definition.
(short for colonel)

I can't explain the rest of the clue.

(Other definitions for col that I've seen before include "Depression among ecologists" , "Low point of saddle between two peaks" , "Officer" , "A depression in mountain range" , "Mustard" .)

I've seen this clue in the Evening Standard.

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