Lucifer (5)
I believe the answer is:
satan
'lucifer' is the definition.
(I know that lucifer can be written as Satan)
This is the entire clue.
(Other definitions for satan that I've seen before include "The devil, Beelzebub" , "Fallen archangel" , "Diabolical figure" , "Fiend" , "Old Nick, Lucifer or Beelzebub" .)