Be adjacent to (4)
'be adjacent to' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abut that I've seen before include "Be next to -- share a boundary" , "Adjoin at one end" , "Adjoin -- 19 (rev)" , "Tuba (anag)" , "Touch, lean on" .)
(Other definitions for neighbour that I've seen before include "Such a one might help out" , "One at your side" , "Person who lives next door" , "One who must live alongside" , "whom you may force into beggary" .)