One in the San Francisco police? (3)
I believe the answer is:
cop
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'in the san francisco police?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'COP' is hidden in the letters of '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" .)