Tall birds (4)
I believe the answer is:
emus
'tall 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" , "Aussie birds" , "Fast, but not high-fliers" , "Large flightless birds of Australia" , "Largest native Australian birds" .)