Bones in back area (6)
I believe the answer is:
sterna
'bones' is the definition.
(sternum is a kind of bone)
'back area' is the wordplay.
'back' becomes 'stern' (I've seen this before).
'area' becomes 'a' (maths abbreviation).
'stern'+'a'='STERNA'
'in' is the link.
(Other definitions for sterna that I've seen before include "Breastbones" , "Formal backbones, Latin plural" , "Bones in front" .)