Old bag of bones (6)
I believe the answer is:
osteal
'of bones' is the definition.
(I've seen this before)
'old bag' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bag' becomes 'steal' (I've seen this before).
'o'+'steal'='OSTEAL'
(Other definitions for osteal that I've seen before include "skeleton may be thus described" , "Relating to bone - so late (anag)" , "Bony" , "made of bone" , "of bones" .)