Shut up. It's only a hundred! (6)
I believe the answer is:
button
'shut up' is the definition.
(thesaurus)
'only a hundred' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'a hundred' becomes 'ton' (I've seen this before).
'but'+'ton'='BUTTON'
'it's' acts as a link.
(Other definitions for button that I've seen before include "Fastener and former F1 driver" , "Electric switch operated by pressing" , "fast driver?" , "Round fastener" , "Fastener used on clothes etc" .)