Cut in knee, say (5)
I believe the answer is:
joint
This clue is a double definition.
'cut' is the first definition.
(joint is a kind of cut)
'knee say' is the second definition.
(knee is a kind of joint)
'in' is the link.
(Other definitions for joint that I've seen before include "Piece of meat - illegal smoke" , "American bar" , "Knee or elbow?" , "Structure by which bones are fitted together" , "Knee is one" .)