Amusing or silly fool (9)
I believe the answer is:
ignoramus
'fool' is the definition.
(thesaurus)
'amusing or silly' is the wordplay.
'silly' indicates an anagram.
'amusing'+'or'='amusingor'
'amusingor' with letters rearranged gives 'IGNORAMUS'.
(Other definitions for ignoramus that I've seen before include "Dunce" , "Dimwit" , "Uninformed sort" , "One who knows nothing" , "unwitting person" .)