Defends joint title (12)
I believe the answer is:
championship
'title' is the definition.
(I know that title can be written as championship)
'defends joint' is the wordplay.
'defends' becomes 'champions' ('champion' can be a synonym of 'defend').
'joint' becomes 'hip joint' (part of body).
'champions'+'hip'='CHAMPIONSHIP'
(Other definitions for championship that I've seen before include "Contest - advocacy" , "title" , "Sporting contest to decide overall winner" .)