Fool is amusing or funny (9)
I believe the answer is:
ignoramus
'fool' is the definition.
(thesaurus)
'amusing or funny' is the wordplay.
'funny' is an anagram indicator.
'amusing'+'or'='amusingor'
'amusingor' anagrammed gives 'IGNORAMUS'.
'is' is the link.
(Other definitions for ignoramus that I've seen before include "A know-nothing" , "I wouldn't know" , "At the present time" , "One lacking knowledge" , "one in a state of bliss?" .)