Fool strangely amusing about old King (9)
I believe the answer is:
ignoramus
'fool' is the definition.
(thesaurus)
'strangely amusing about old king' is the wordplay.
'strangely' indicates anagramming the letters.
'about' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'king' becomes 'r' (r is abbreviation for rex).
'amusing' with letters rearranged gives 'ignamus'.
'o'+'r'='or'
'ignamus' enclosing 'or' is 'IGNORAMUS'.
(Other definitions for ignoramus that I've seen before include "Stupid person" , "At the present time" , "Dimwit" , "Dunce" , "One who knows nothing" .)