If cruel, can be the devil! (7)
I believe the answer is:
lucifer
'be the devil' is the definition.
(thesaurus)
'if cruel can' is the wordplay.
'can' indicates anagramming the letters (I've seen 'canned' mean this (canned can mean drunk)).
'if'+'cruel'='ifcruel'
'ifcruel' anagrammed gives 'LUCIFER'.
(Other definitions for lucifer that I've seen before include "Venus" , "Devilish sort of match" , "Beelzebub, Old Nick" , "once a match for > Dickens" , "Rebellious angel; a match" .)