Fool is amusing or ridiculous (9)
I believe the answer is:
ignoramus
'fool' is the definition.
(thesaurus)
'amusing or ridiculous' is the wordplay.
'ridiculous' indicates anagramming the letters.
'amusing'+'or'='amusingor'
'amusingor' with letters rearranged gives 'IGNORAMUS'.
'is' acts as a link.
(Other definitions for ignoramus that I've seen before include "One could be blissful it's said" , "Uninformed sort" , "One is not intelligent" , "Stupid person" , "Fool" .)