Rhea's relative (3)
I believe the answer is:
'rhea's relative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for emu that I've seen before include "it can't get off the ground" , "Ostrich-like bird" , "Native Australian bird" , "Australian bird related to the ostrich" , "Fast runner in Oz" .)