Legendary knight's wife leaving joint in vehicle (8)
I believe the answer is:
tristram
'legendary knight's' is the definition.
'in vehicle' is the wordplay.
'in' becomes 'tris' (I can't justify this - if you can you should give a lot more credence to this answer).
'vehicle' becomes 'tram' (I've seen this before).
'tris'+'tram'='TRISTRAM'
'wife leaving joint' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tristram that I've seen before include "' - - Shandy' Sterne novel" , "K" , "Arthurian knight - Shandy" , "Cornish knight of the Round Table" .)