Big bird's bone uncovered (3)
I believe the answer is:
emu
'big bird's' is the definition.
(I've seen this in another clue)
'bone uncovered' is the wordplay.
'bone' becomes 'femur' (femur is a kind of bone).
'uncovered' says to take the centre.
The middle of 'femur' is 'EMU'.
(Other definitions for emu that I've seen before include "Aussie runner" , "that doesn't get off the ground" , "Ostrich-like bird" , "it'll never fly!" , "Parky's nemesis" .)