Middle of the leg (4)
(Other definitions for knee that I've seen before include "One has a cap" , "middle part of leg" , "Capped joint?" , "Where the femur meets the tibia" , "joint input" .)
(Other definitions for kneecap that I've seen before include "Joint protection" , "The informal patella (7)" , "Bone" , "one going on bender?" , "part of joint" .)