Big birds Down Under (4)
I believe the answer is:
emus
'big birds down under' 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 "Running birds" , "Muse about flightless Australian birds" , "Fast, but not high-fliers" , "Largest native Australian birds" , "Aussie birds" .)