Devil, Santa in disguise! (5)
I believe the answer is:
satan
'devil' is the definition.
('Satan' can be a synonym of 'devil')
'santa in disguise' is the wordplay.
'in disguise' indicates an anagram.
'santa' with letters rearranged gives 'SATAN'.
(Other definitions for satan that I've seen before include "Old Nick, Lucifer or Beelzebub" , "Fiend" , "Prince of Darkness" , "The devil, Beelzebub" , "Fallen archangel" .)