Fool can be amusing or stupid (9)
I believe the answer is:
ignoramus
'fool' is the definition.
(thesaurus)
'amusing or stupid' is the wordplay.
'stupid' indicates anagramming the letters (in a stupid order).
'amusing'+'or'='amusingor'
'amusingor' is an anagram of 'IGNORAMUS'.
'can be' is the link.
(Other definitions for ignoramus that I've seen before include "At the present time" , "A know-nothing" , "unwitting person" , "Stupid person" , "I wouldn't know" .)