Something between a knee and an ankle (6)
I believe the answer is:
fibula
'ankle' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe there's a link between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fibula that I've seen before include "Outer bone in the calf" , "Bone of the lower leg" , "A lower-leg bone" , "Human leg-bone" .)