It's in one's bones (6)
I believe the answer is:
marrow
'bones' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for marrow that I've seen before include "Soft tissue in bones" , "Fatty stuff in bones or large fruit" , "Large vegetable gourd or stuff in bones" , "Vegetable; bone filling" , "Large gourd fruit or stuff in bone cavities" .)