Something deadly in Xmas pud (3)
I believe the answer is:
asp
'something deadly' is the definition.
'in xmas pud' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ASP' can be found hidden inside 'xmas pud'.
(Other definitions for asp that I've seen before include "Pa's Cleopatra's killer" , "It proved deadly for Cleopatra" , "Cleopatra's supposed nemesis" , "Cold-blooded killer" , "Small viper" .)