It's "the word"
I believe the answer is:
mum
'it's the word' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mum that I've seen before include "Keep it secret from mother?" , "old lady" , "Mother - she's the word for secrecy" , "Keep quiet, mother" , "Informal palindromic mother" .)