A force for good (6)

I believe the answer is:

police

I'm a little stuck... Click here to teach me more about this clue!

'a force' is the definition.
(I know that police is a type of force)

I can't explain the rest of the clue.

(Other definitions for police that I've seen before include "Keep watch on" , "Constabulary" , "Civil force" , "... custodians of law and order" , "Law enforcers" .)

I've seen this clue in the Evening Standard.

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