Share a side (4)
I believe the answer is:
abut
'share a side' 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 "Lie adjacent to" , "Adjoin -- 19 (rev)" , "Neighbour" , "Be next to -- share a boundary" , "Touch upon" .)