If cruel, could be the devil! (7)
I believe the answer is:
lucifer
'the devil' is the definition.
(synonyms)
'if cruel could be' is the wordplay.
'could be' indicates anagramming the letters.
'if'+'cruel'='ifcruel'
'ifcruel' is an anagram of 'LUCIFER'.
(Other definitions for lucifer that I've seen before include "Match - Satan" , "Devil - match!" , "Satan, the Devil" , "from the abyss?" , "Light" .)