Theirs is a service station (6)

I believe the answer is:

police

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'station' is the definition.
(police station is a kind of station)

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

(Other definitions for police that I've seen before include "Emergency service" , "Bobby" , "Crime fighters" , "Limbs of the law" , "9a 19d" .)

I've seen this clue in the Evening Standard.

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