Long-extinct bird (4)
I believe the answer is:
dodo
'long-extinct bird' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dodo that I've seen before include "E [EXTINCT] bird" , "Proverbially extinct bird" , "Native of Mauritius" , "Idiot" , "Proverbially dead bird" .)