Fine bird with rook bone (5)
I believe the answer is:
femur
'bone' is the definition.
(I know that femur is a bone)
'fine bird with rook' is the wordplay.
'fine' becomes 'f' (abbreviation e.g. condition of old coins, books).
'bird' becomes 'emu' (emu is a type of bird).
'with' says to put letters next to each other.
'rook' becomes 'R' (chess abbreviation).
'f'+'emu'+'r'='FEMUR'
(Other definitions for femur that I've seen before include "bit of leg" , "Longest bone in the human body" , "Part of leg" , "Bone in leg" , "Thigh bone" .)