Part of the body to slap (4)
I believe the answer is:
knee
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">part of the body to slap</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 "Capped joint?" , "Genuflector?" , "Major joint in leg" , "may present problem to maid" , "Hinged joint" .)