Bones in back and front of arm (6)
I believe the answer is:
sterna
'bones' is the definition.
(more than one sternum)
'back and front of arm' is the wordplay.
'back' becomes 'stern' (stern means the back of a boat).
'and' is a charade indicator (letters next to each other).
'front of' indicates taking the first letters (the letter at the front of the word).
The first letter of 'arm' is 'a'.
'stern'+'a'='STERNA'
'in' acts as a link.
(Other definitions for sterna that I've seen before include "Formal backbones, Latin plural" , "Breastbones" , "Bones in front" .)