Diner slice (3)
I believe the answer is:
pie
'diner slice' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pie that I've seen before include "Dish baked in pastry-lined pan" , "In the sky, it's an empty promise" , "Dish baked in an oven, with pastry or mashed potato topping" , "Bird; pastry" , "welcome luxury" .)