Diabolic comedian fluffed a line (10)

I believe the answer is:

demoniacal

'diabolic' is the definition.
(thesaurus)

'comedian fluffed a line' is the wordplay.
'fluffed' indicates an anagram.
'line' becomes 'l' (used when specifying particular lines from a poem).
'comedian' with letters rearranged gives 'demoniac'.
'demoniac'+'a'+'l'='DEMONIACAL'

(Another definition for demoniacal that I've seen is " Fiendish".)

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