Act, a sin that might be diabolical (7)
I believe the answer is:
satanic
'diabolical' is the definition.
(I know that diabolical can be written as satanic)
'act a sin that might be' is the wordplay.
'that might be' indicates an anagram.
'act'+'a'+'sin'='actasin'
'actasin' with letters rearranged gives 'SATANIC'.
(Other definitions for satanic that I've seen before include "Fiendish" , "Hellish" , "Very evil and cruel" , "Bad" , "Diabolic" .)