Cast in a devilish form (7)
I believe the answer is:
satanic
'cast in a devilish form' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'cast in a devilish' is the wordplay.
'devilish' indicates anagramming the letters (a devious or devilish spelling).
'cast'+'in'+'a'='castina'
'castina' anagrammed gives 'SATANIC'.
Can you help me to learn more?
(Other definitions for satanic that I've seen before include "Devil-like" , "Demonic" , "Hellish" , "Fiendish" , "Devilish (as Blake's 18)" .)