Kay's born in low joint (4)
I believe the answer is:
knee
'low joint' is the definition.
(joint of the body)
'kay's born' is the wordplay.
'kay' becomes 'k'.
'born' becomes 'nee' ('nee' is 'born' in French).
'k'+'nee'='KNEE'
'in' acts as a link.
(Other definitions for knee that I've seen before include "Where the femur meets the tibia" , "Lower limb joint" , "Leg?joint" , "One has a cap" , "joint input" .)