Valid if you're a member of the police (2,5)

I believe the answer is:

in force

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'valid' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for in force that I've seen before include "Bobby is" , "with large numbers" , "(Of law) legally binding" , "Recruited for the police" , "Legally operative" .)

I've seen this clue in the Evening Standard.

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