Evil one cruel if disturbed (7)
I believe the answer is:
'evil one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'cruel if disturbed' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'cruelif' with letters rearranged gives 'LUCIFER'.
Can you help me to learn more?
(Other definitions for lucifer that I've seen before include "Rebellious angel; a match" , "Prince of Darkness" , "Satan, the Dark Angel" , "Devil - match!" , "Satan, the Devil" .)