Extinct bird’s endless complaint (3)
I believe the answer is:
moa
'extinct bird's' is the definition.
(extinct flightless bird of New Zealand)
'endless complaint' is the wordplay.
'endless' means to remove the last letter.
'complaint' becomes 'moan' (a moan is a complaint or objection).
'moan' with its final letter taken off is 'MOA'.
(Other definitions for moa that I've seen before include "bird that became extinct" , "Bird that died out" , "Bird no longer" , "Extinct flightless bird of New Zealand" , "Gigantic extinct bird" .)