Joint protected by the patella (4)
I believe the answer is:
knee
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">joint protected by the patella</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for knee that I've seen before include "Thigh/shin connector" , "Hinge between the thigh and the shin" , "Leg?joint" , "Technical name for this joint is genu" , "It jerks reactively" .)