Proverbially extinct bird (4)
I believe the answer is:
dodo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">proverbially extinct bird</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)