It's nonsense to call for silence (4)
I believe the answer is:
tosh
'it's nonsense' is the definition.
(tosh is a kind of nonsense)
'to call for silence' is the wordplay.
'call for silence' becomes 'sh' ('sh' is used to tell someone to be quiet).
'to'+'sh'='TOSH'
(Other definitions for tosh that I've seen before include "Boloney" , "garbage'" , "Scottish chum" , "Humbug" , "Intimate" .)