Silent; mother (3)
I believe the answer is:
mum
This clue is a double definition.
'silent' is the first definition.
(keep mum)
'mother' is the second definition.
(informal term for mother)
(Other definitions for mum that I've seen before include "Mother - she's the word for secrecy" , "Palindromic mater" , "Tight-lipped" , "Parent (who's kept or is the word!)" , "Keep it secret from mother?" .)