Australian flightless birds (4)
I believe the answer is:
emus
'australian flightless birds' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for emus that I've seen before include "Muse about flightless Australian birds" , "laboratory units" , "Running birds" , "Large flightless birds of Australia" , "Largest native Australian birds" .)