Rear or reverse (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">rear</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">reverse</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 "Support (from behind?)" , "Golf holes 10-18" , "ago" , "Stern" , "Series of popular time-travel movies" .)