An act is horribly evil (7)
I believe the answer is:
satanic
'evil' is the definition.
('satanic' can be similar in meaning to 'evil')
'an act is horribly' is the wordplay.
'horribly' indicates an anagram.
'an'+'act'+'is'='anactis'
'anactis' is an anagram of 'SATANIC'.
(Other definitions for satanic that I've seen before include "Diabolic" , "Relating to the devil" , "Demonic" , "Very evil and cruel" , "Nick's" .)