Mother is silent (3)
I believe the answer is:
mum
This clue is a double definition.
'mother' is the first definition.
(informal term for mother)
'silent' is the second definition.
(synonyms)
'is' acts as a link.
(Other definitions for mum that I've seen before include "Silence" , "Keep quiet, mother" , "Woman who's delivered" , "old lady" , "Informal palindromic mother" .)