Share a boundary with (4)
I believe the answer is:
abut
'share a boundary with' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for abut that I've seen before include "Lie adjacent to" , "Adjoin, press up against" , "End or lean against, say a building" , "Impinge" , "Adjoin -- 19 (rev)" .)