One in the San Francisco police (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 understand how one could define the other.

'in the san francisco police?' is the wordplay.
'in' indicates a hidden word.
'COP' can be found hidden inside 'san francisco police'.

Can you help me to learn more?

(Other definitions for cop that I've seen before include "catch" , "An American policeman" , "Policeman (colloq.)" , "Police officer (informal)" , "Member of the police force and an arrest" .)

I've seen this clue in the Evening Standard.

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