An eye for a Cockney's Christmas treat? (5,3)
I believe the answer is:
mince pie
I believe this clue is a double definition.
'an eye for a cockney's' is the first definition.
('mince pies' is Cockney rhyming slang for 'eyes')
'christmas treat?' is the second definition.
(I've seen this before)
(Other definitions for mince pie that I've seen before include "Fruit-filled tart" , "Eye" , "Christmas treat" , "Christmas food item" , "Christmas fare" .)