Silly joke by amateur (4)
I believe the answer is:
gaga
'silly' is the definition.
(thesaurus)
'joke by amateur' is the wordplay.
'joke' becomes 'gag' (gagging is a kind of joking).
'by' means one lot of letters go next to another.
'amateur' becomes 'a'.
'gag'+'a'='GAGA'
(Other definitions for gaga that I've seen before include "crackers" , "Senile or crazy in repetitive way" , "dotty" , "Lady who sings" , "US singer Lady ___" .)