King once called joint (4)
I believe the answer is:
knee
'joint' is the definition.
(I know that knee is a type of joint)
'king once called' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'once called' becomes 'nee' (I've seen this in another clue).
'k'+'nee'='KNEE'
(Other definitions for knee that I've seen before include "Capped joint?" , "Leg hinge" , "one is bent on submission" , "Where the femur meets the tibia" , "One has a cap" .)