Is she speechless? (3)

I believe the answer is:

mum

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'speechless?' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for mum that I've seen before include "Informal palindromic mother" , "Keep quiet, mother" , "Tight-lipped" , "member of the family" , "Parent - shtoom" .)

I've seen this clue in the Evening Standard.

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