Possession of unopened tranquillisers and joint (9)
I believe the answer is:
ownership
'possession of' is the definition.
(I've seen this before)
'tranquillisers and joint' is the wordplay.
'tranquillisers' becomes 'owners' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' says to put letters next to each other.
'joint' becomes 'hip' (part of body).
'owners'+'hip'='OWNERSHIP'
'unopened' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ownership that I've seen before include "The right of possessing something" , "Legal title" , "Proprietorship, possession" , "Proprietary right of possession" .)