Bony, very backward duck (6)
I believe the answer is:
osteal
'bony' is the definition.
('osteal' can be a synonym of 'bony')
'very backward duck' is the wordplay.
'very' becomes 'so' (I've seen this in other clues).
'backward' says the letters should be written in reverse.
'duck' becomes 'teal' (teal is a kind of duck**).
'so' written backwards gives 'os'.
'os'+'teal'='OSTEAL'
(Other definitions for osteal that I've seen before include "of bones" , "Relating to bone - so late (anag)" , "skeleton may be thus described" , "Bony" , "made of bone" .)