This shows possession of the proprietor's joint! (9)
I believe the answer is:
ownership
'this shows possession' is the definition.
'ownership' can be an answer for 'possession' (ownership is a kind of possession). I am not sure about the 'this shows' bit.
'proprietor's joint' is the wordplay.
'proprietor' becomes 'owner' (synonyms).
'joint' becomes 'ship' (I've seen this in other clues).
'owner'+'ship'='OWNERSHIP'
'of the' acts as a link.
Can you help me to learn more?
(Other definitions for ownership that I've seen before include "The right of possessing something" , "Legal title" , "Proprietary right of possession" , "Proprietorship, possession" .)