Bird that lays the largest egg (7)
I believe the answer is:
ostrich
'bird that lays the largest egg' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ostrich that I've seen before include "The head-in-the-sand bird" , "Denier" , "non-flyer?" , "Bird burying its head in the sand" , "The biggest bird, can't fly" .)