Silly joke on the origin of America (4)
I believe the answer is:
gaga
'silly' is the definition.
(thesaurus)
'joke on the origin of america' is the wordplay.
'joke' becomes 'gag' ('gag' can be a synonym of 'joke'**).
'on' is a charade indicator (letters next to each other).
'the origin of' says to take the initial letters.
The initial letter of 'america' is 'a'.
'gag'+'a'='GAGA'
(Other definitions for gaga that I've seen before include "Doddery" , "Slightly mad (colloq.)" , "In senile dotage" , "Bats" , "Lady < > in a senile" .)