Share a boundary (4)
I believe the answer is:
abut
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">share a boundary</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for abut that I've seen before include "Touch (side of next building)" , "Adjoin at one end" , "Adjoin, press up against" , "Adjoin, be contiguous to" , "Lie next to" .)