Share a boundary (4)
I believe the answer is:
abut
'share a boundary' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)