It's the word for 'silent' (3)
I believe the answer is:
mum
I believe this is a double definition.
'it's the word' is the first definition.
(I've seen this in another clue)
'silent' is the second definition.
(keep mum)
'for' is the link.
(Other definitions for mum that I've seen before include "Quiet; relative" , "Parent - shtoom" , "old lady" , "Mother - she's the word for secrecy" , "Mother (colloq.)" .)