Big flightless bird (3)
I believe the answer is:
emu
'big flightless bird' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for emu that I've seen before include "Big bird" , "it'll never fly!" , "Fast runner in Oz" , "Ostrich-like bird" , "Long-necked bird" .)