Does dad keep her quiet? (3)

I believe the answer is:

mum

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

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

(Other definitions for mum that I've seen before include "Keep quiet, mother" , "Silent; parent" , "Parent (who's kept or is the word!)" , "member of the family" , "Mother (colloq.)" .)

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