Down Under birds (4)
I believe the answer is:
emus
'down under 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 "Large flightless birds of Australia" , "Running birds" , "Muse about flightless Australian birds" , "Largest native Australian birds" , "laboratory units" .)