Boundary, margin (6)
I believe the answer is:
border
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">boundary</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that border is a type of boundary)</span><br/ ><br/ >'<span class="explanation-format__original">margin</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that margin can be written as border)</span><br/ ><br/ ></span>
(Other definitions for border that I've seen before include "Rim" , "Line that indicates a boundry" , "Type of sheepdog" , "BLINK/BRINK" , "Quantity; song" .)