Element in bones (7)
I believe the answer is:
calcium
'element in bones' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for calcium that I've seen before include "Chemical element in bones, basis of lime" , "White metallic element found in most animals and plants, Ca" , "Bone constituent" , "Atomic number 20" , "Element, component of bones and teeth" .)