Santa's new demon (5)
I believe the answer is:
satan
'demon' is the definition.
(Satan means the devil)
'santa's new' is the wordplay.
'new' indicates an anagram.
'santa' with letters rearranged gives 'SATAN'.
(Other definitions for satan that I've seen before include "Prince of Darkness" , "Fallen archangel" , "arch-fiend" , "Fiend" , "Old Nick, Lucifer or Beelzebub" .)