One in the San Francisco PD? (3)

I believe the answer is:

cop

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'in the san francisco pd?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'COP' can be found hidden inside 'san francisco pd'.

Can you help me to learn more?

(Other definitions for cop that I've seen before include "catch" , "An American policeman" , "Policeman (sl)" , "Short policeman" , "Policeman (colloq.)" .)

I've seen this clue in the Evening Standard.

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