Border, boundary (4)
I believe the answer is:
edge
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">border</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">boundary</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for edge that I've seen before include "Guitarist with U2" , "Margin, fringe" , "Margin, outside limit" , "Rim or sharp side of knife" , "Steal" .)