Border on (4)
I believe the answer is:
abut
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">border on</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for abut that I've seen before include "Touch, lean on" , "Be next to -- share a boundary" , "Meet" , "Lie adjacent to" , "Adjoin, press up against" .)