Fool not wholly benign or amusing (9)
I believe the answer is:
ignoramus
'fool' is the definition.
(thesaurus)
'not wholly benign or amusing' is the wordplay.
'not wholly' says the answer is hidden in the clue (I've seen this in other clues).
'IGNORAMUS' is hidden in the letters of 'benign or amusing'.
(Other definitions for ignoramus that I've seen before include "One lacking knowledge" , "Fool" , "Ignorant person" , "Stupid person" , "One could be blissful it's said" .)