Fellow of intelligence with a mental disorder (9)
I believe the answer is:
gentleman
'fellow' is the definition.
(thesaurus)
'intelligence with a mental disorder' is the wordplay.
'intelligence' becomes 'gen'.
'with' is an insertion indicator.
'disorder' is an anagram indicator (I've seen 'diseased' mean this (I've seen this in other clues)).
'mental' with letters rearranged gives 'ntlema'.
'gen' going around 'ntlema' is 'GENTLEMAN'.
'of' acts as a link.
(Other definitions for gentleman that I've seen before include "A man of refinement" , "Polite, kind or upper-class fellow" , "-'s -; - farmer" , "Courteous chap" , "Courteous male" .)