Bad comedian is diabolical (8)
I believe the answer is:
demoniac
'diabolical' is the definition.
(thesaurus)
'bad comedian' is the wordplay.
'bad' indicates an anagram.
'comedian' with letters rearranged gives 'DEMONIAC'.
'is' acts as a link.
(Other definitions for demoniac that I've seen before include "man possessed" , "Diabolical" , "one that's potentially evil" , "Comedian of devilish sort 98)" , "Person possessed by an evil spirit" .)