Ice cream with the last piece of Christmas cake (5)
I believe the answer is:
scone
'cake' is the definition.
(scone is a type of cake)
'ice cream with the last piece of christmas' is the wordplay.
'ice cream' becomes 'cone' (cone can refer to an ice cream cone).
'with' says to put letters next to each other.
'the last piece of' says to take the final letters.
The last letter of 'christmas' is 's'.
'cone' put after 's' is 'SCONE'.
(Other definitions for scone that I've seen before include "A little cake" , "in which a stone was once found?" , "The Stone of -----, returned to Scotland in 1996 after 700 years" , "Griddle-cake" , "Product of baker" .)