Half the diameter of a bone (6)
I believe the answer is:
radius
I believe this is a double definition.
'half the diameter' is the first definition.
(I've seen this in another clue)
'a bone' is the second definition.
(radius is a kind of bone)
'of' acts as a link.
(Other definitions for radius that I've seen before include "Centre-circumference length" , "from centre to boundary" , "Bone of forearm - distance from centre" , "Distance from centre to circumference of circle" , "Human forearm bone" .)