Not saying a thing (3)
I believe the answer is:
mum
'not saying a thing' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mum that I've seen before include "Mother (colloq.)" , "old lady" , "Informal palindromic mother" , "Parent - shtoom" , "Parent (who's kept or is the word!)" .)