Demon among demons -- terrifying (7)
I believe the answer is:
monster
'demon' is the definition.
('monster' can be a synonym of 'demon')
'among demons terrifying' is the wordplay.
'among' indicates the answer is hidden within the clue.
'MONSTER' is hidden within 'demons terrifying'.
(Other definitions for monster that I've seen before include "Behemoth" , "Large, mytical creature" , "Very big" , "Fabulous creature" , "Fabled giant animal" .)