It's nonsense to be quiet (4)
I believe the answer is:
tosh
'it's nonsense' is the definition.
(tosh is a kind of nonsense)
'to be quiet' is the wordplay.
'be quiet' becomes 'sh'.
'to'+'sh'='TOSH'
(Other definitions for tosh that I've seen before include "Slang nonsense" , "Nonsense (colloq.)" , "garbage'" , "mate!" , "Scottish chum" .)