Flightless Australian birds (4)
I believe the answer is:
emus
'flightless australian birds' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for emus that I've seen before include "Largest native Australian birds" , "Aussie birds" , "Muse about flightless Australian birds" , "Large flightless birds of Australia" , "Fast, but not high-fliers" .)