Feast where diner consumes last of eggs (6)
I believe the answer is:
easter
'feast' is the definition.
(I've seen this before)
'diner consumes last of eggs' is the wordplay.
'diner' becomes 'eater' (to dine is to eat a meal).
'consumes' means one lot of letters goes inside another.
'last of' indicates one should take the final letters.
The last letter of 'eggs' is 's'.
'eater' placed around 's' is 'EASTER'.
'where' acts as a link.
(Other definitions for easter that I've seen before include "Time to celebrate" , "8 9 [PACIFIC ISLAND]" , "Monday of 1916 Rising" , "Parade; egg; lily" , "A terse sort of Spring festival?" .)