Plain clothes men in the incident room (3)

I believe the answer is:

cid

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'plain clothes men' is the definition.
The definition suggests a singular noun which matches the answer.

'in the incident room' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CID' can be found hidden inside 'incident room'.

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I've seen this clue in the Evening Standard.

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