Proverbially extinct bird (4)
I believe the answer is:
dodo
'proverbially extinct bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dodo that I've seen before include "Ultra-conservative" , "Bird (whose style of very out of fashion)" , "Old-fashioned person" , "one very dead!" , "Mauritian bird no more" .)