Cassowary relatives (4)
I believe the answer is:
emus
'cassowary relatives' 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 "Fast, but not high-fliers" , "laboratory units" , "Running birds" , "Largest native Australian birds" , "Large flightless birds of Australia" .)