Father describing flightless bird's bone (5)
I believe the answer is:
femur
'bone' is the definition.
(I know that femur is a type of long bone)
'father describing flightless bird's' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'flightless bird' becomes 'emu' (emu is a kind of flightless bird).
'fr' enclosing 'emu' is 'FEMUR'.
(Other definitions for femur that I've seen before include "Part of leg" , "Large bone" , "Thighbone" , "bit of leg" , "Longest bone in the human body" .)