"Hard-boiled" breakfast (3)
I believe the answer is:
egg
'hard-boiled breakfast' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for egg that I've seen before include "Cooked breakfast staple" , "Oval item" , "Food from a hen" , "Mine (slang)" , "It comes before or after the chicken?" .)