Kay previously called for joint (4)
I believe the answer is:
knee
'for joint' is the definition.
(knee joint is a kind of joint)
'kay previously called' is the wordplay.
'kay' becomes 'k'.
'previously called' becomes 'nee' (French for 'born', used before a previous surname).
'k'+'nee'='KNEE'
(Other definitions for knee that I've seen before include "Human joint" , "Joint protected by the patella" , "Hinged joint" , "for going on begging" , "Hinge between the thigh and the shin" .)