Diabolical cast in a play (7)
I believe the answer is:
satanic
'diabolical' is the definition.
(I know that diabolical can be written as satanic)
'cast in a play' is the wordplay.
'play' indicates anagramming the letters.
'cast'+'in'+'a'='castina'
'castina' anagrammed gives 'SATANIC'.
(Other definitions for satanic that I've seen before include "wicked" , "Devil-like" , "Hellish" , "like Mephisto" , "Bad" .)