Rubbish? You have to keep quiet! (4)
I believe the answer is:
tosh
'rubbish?' is the definition.
(both can mean nonsense)
'to keep quiet' is the wordplay.
'keep quiet' becomes 'sh' (as in shushing someone).
'to'+'sh'='TOSH'
'you have' acts as a link.
(Other definitions for tosh that I've seen before include "Pal" , "garbage'" , "Scottish chum" , "Rubbish, nonsense" , "Nonsense; shot (anag.)" .)