Diabolical and awful act, a sin (7)
I believe the answer is:
satanic
'diabolical' is the definition.
('satanic' can be a synonym of 'diabolical')
'awful act a sin' is the wordplay.
'awful' indicates an anagram.
'act'+'a'+'sin'='actasin'
'actasin' with letters rearranged gives 'SATANIC'.
'and' is the link.
(Other definitions for satanic that I've seen before include "Nick's" , "Devilish (as Blake's 18)" , "Diabolic" , "wicked" , "Supremely evil" .)