Jumbo order for one who's really hungry (8)
I believe the answer is:
elephant
I believe this is a double definition.
'jumbo' is the first definition.
(Jumbo was an elephant who lived in London Zoo)
'order for one who's really hungry' is the second definition.
("I'm so hungry I could eat an elephant")
(Other definitions for elephant that I've seen before include "Size of paper" , "Largest living land animal" , "Get this animal from Pat and Helen" , "Large pachyderm" , "Tusked mammal" .)