Knee bone (7)
I believe the answer is:
patella
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">knee bone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for patella that I've seen before include "bit of body usually under trousers!" , "Kneecap (anatomy)" , "positioned on each knee" , "Joint cover" , "The formal kneecap" .)