Dummy's perch (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dummy's perch</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for knee that I've seen before include "It jerks reactively" , "Leg hinge" , "Technical name for this joint is genu" , "Leg bender" , "Where the femur meets the tibia" .)