In the RAF, short missions (3)

I believe the answer is:

ops

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'in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for ops that I've seen before include "military activity" , "Military operations" , "Operations room" , "Military manoeuvres (abbr)" , "medical procedures" .)

I've seen this clue in the Evening Standard.

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