Something poisonous in Christmas pudding (3)
I believe the answer is:
asp
'something poisonous' is the definition.
'in christmas pudding' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ASP' can be found hidden inside 'christmas pudding'.
(Other definitions for asp that I've seen before include "snake appears" , "Snake, killed Cleopatra" , "something at the zoo" , "Snake found in spa" , "This Egyptian killer" .)