Quiet member of the family? (3)
I believe the answer is:
This clue is a double definition.
'quiet' is the first definition.
'member of the family?' is the second definition.
(I've seen this before)
(Other definitions for mum that I've seen before include "Silence" , "member of the family" , "Informal palindromic mother" , "Mother - she's the word for secrecy" , "Quiet; relative" .)