Support; rear surface (4)
I believe the answer is:
back
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">support</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(to back is to support or endorse)</span><br/ ><br/ >'<span class="explanation-format__original">rear surface</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for back that I've seen before include "See 12" , "Support - football player" , "Fashionable again" , "Help!" , "part of the body" .)