Adjoin (4)
I believe the answer is:
abut
'adjoin' is the definition.
(abutting is a kind of adjoining)
This is the entire clue.
(Other definitions for abut that I've seen before include "Touch, lean on" , "Neighbour" , "Adjoin, touch" , "Adjoin at one end" , "Be next to -- share a boundary" .)