Endless complaint from one unable to fly (3)
I believe the answer is:
moa
'one unable to fly' is the definition.
(type of flightless bird)
'endless complaint' is the wordplay.
'endless' means to remove the last letter.
'complaint' becomes 'moan' (I've seen this before).
'moan' with its last letter removed is 'MOA'.
'from' acts as a link.
(Other definitions for moa that I've seen before include "was hunted by Maori" , "Bird's gone for ever" , "Bird lost" , "Gigantic extinct bird" , "Extinct large flightless NZ bird" .)