It's an affront
I believe the answer is:
slap
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">it's an affront</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 slap that I've seen before include "Hit with the palm" , "Hit with a flat hand" , "Cuff" , "A blow among pals" , "Strike (in the face)" .)