One is surrounded by numbers of detectives (3)

I believe the answer is:

cid

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'detectives' is the definition.
(abbreviation for Criminal Investigation Department)

'one is surrounded by numbers' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is surrounded by' means one lot of letters goes inside another.
'numbers' becomes 'cd' (I can't explain this - if you can you should give a lot more credence to this answer).
'i' placed within 'cd' is 'CID'.

'of' acts as a link.

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I've seen this clue in the Evening Standard.

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