Skeletal bird, previously overlarge (6)
I believe the answer is:
osteal
'skeletal' is the definition.
(osteal means relating to bones)
'bird previously overlarge' is the wordplay.
'bird' becomes 'teal' (type of duck).
'previously' means one lot of letters go next to another.
'overlarge' becomes 'OS' (abbreviation for outsize).
'teal' after 'os' is 'OSTEAL'.
(Other definitions for osteal that I've seen before include "of bones" , "made of bone" , "skeleton may be thus described" , "Bony" , "Relating to bone - so late (anag)" .)