Fiendish, infernal (7)
I believe the answer is:
demonic
I believe this is a double definition.
'fiendish' is the first definition.
(I know that fiendish can be written as demonic)
'infernal' is the second definition.
(I know that infernal can be written as demonic)
(Other definitions for demonic that I've seen before include "Inspired as if by a fiend" , "diabolical" , "Furious" , "Cruel like the devil" , "Like a devil or evil spirit" .)