Only bird with calf muscles (3)
I believe the answer is:
emu
'only bird with calf muscles' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for emu that I've seen before include "Extinct bird" , "Flightless bird of Australia, like ostrich" , "it'll never fly!" , "Australian bird related to the ostrich" , "Large Australian flightless bird" .)