Keep her quiet! (3)

I believe the answer is:

mum

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'quiet' is the definition.
(to keep mum is to stay quiet)

I can't explain the remainder of the clue.

(Other definitions for mum that I've seen before include "(Keep) silent; mother" , "Parent (who's kept or is the word!)" , "Woman who's delivered" , "Quiet; relative" , "Mother (colloq.)" .)

I've seen this clue in the Evening Standard.

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