Something to eat that provides constant energy (3)
I believe the answer is:
pie
'something to eat' is the definition.
(pie is a kind of food)
'constant energy' is the wordplay.
'constant' becomes 'pi' (mathematical constant).
'energy' becomes 'e' (physics symbol as in E=mc2).
'pi'+'e'='PIE'
'that provides' is the link.
(Other definitions for pie that I've seen before include "Crusty dish" , "where blackbirds sang" , "Filled pastry" , "Dish baked in pastry-lined pan" , "Pecan or blueberry dish?" .)