Extinct animal (4)
I believe the answer is:
dodo
'extinct animal' is the definition.
'dodo' can be an answer for 'animal' (dodo is a kind of animal). I also know that 'extinct' relates to this answer.
This is the entire clue.
(Other definitions for dodo that I've seen before include "Proverbially extinct bird" , "bird long dead" , "Old-fashioned person" , "Old bird" , "one very dead!" .)