Large, flightless birds (4)
I believe the answer is:
emus
'large flightless birds' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for emus that I've seen before include "Muse about flightless Australian birds" , "Running birds" , "Large flightless birds of Australia" , "Fast, but not high-fliers" , "laboratory units" .)