Silence a crazy (4)
I believe the answer is:
gaga
'crazy' is the definition.
(thesaurus)
'silence a' is the wordplay.
'silence' becomes 'gag' (gagging is a kind of silencing).
'gag'+'a'='GAGA'
(Other definitions for gaga that I've seen before include "bananas" , "Slangily crazy, off one's head" , "fatuous?" , "Barmy joke, third in cracker" , "Slightly mad (colloq.)" .)