Act a sin out? Diabolical (7)
I believe the answer is:
satanic
'diabolical' is the definition.
(I know that diabolical can be written as satanic)
'act a sin out?' is the wordplay.
'out?' is an anagram indicator (I've seen 'out' mean this (out can mean wrong or inaccurate)).
'act'+'a'+'sin'='actasin'
'actasin' anagrammed gives 'SATANIC'.
(Other definitions for satanic that I've seen before include "Demonic" , "Hellish" , "Supremely evil" , "Nick's" , "wicked" .)