Fast-running bird (3)
(Other definitions for emu that I've seen before include "European project" , "Ratite" , "Long-necked bird" , "Flightless bird of Australia, like ostrich" , "Australian non-flyer" .)
'fast-running bird' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ostrich that I've seen before include "Large flightless bird of Africa" , "Largest extant bird" , "Largest non-flying bird" , "Denier" , "Fast-running African flightless bird" .)