Bone cut by knife (8,5)
I believe the answer is:
shoulder blade
'bone' is the definition.
(I know that shoulder blade is a type of bone)
'cut by knife' is the wordplay.
'cut' becomes 'shoulder' (shoulder is a kind of cut).
'by' means one lot of letters go next to another.
'knife' becomes 'blade' (type of blade).
'shoulder'+'blade'='SHOULDER-BLADE'
(Other definitions for shoulder blade that I've seen before include "Scapula" , "joint?" , "bone" .)