Before taking vitamin, endlessly gnaws into part of joint (9)
I believe the answer is:
anklebone
'before taking vitamin endlessly gnaws' is the definition.
I can't tell whether this definition defines the answer.
'into part of joint' is the wordplay.
'into' becomes 'ebon' (I can't justify this - if you can you should believe this answer much more).
'part of' means one lot of letters goes inside another (part of the inside).
'joint' becomes 'ankle' (joint of the body).
'ebon' inserted within 'ankle' is 'ANKLEBONE'.
Can you help me to learn more?
(Another definition for anklebone that I've seen is " Talus".)