Be next to (6)
'be next to' is the definition.
(to adjoin is to be next to something)
This is the entire clue.
(Other definitions for adjoin that I've seen before include "Of building, be situated next to" , "Be next to, say building" , "Abut, border" , "Neighbour" , "Lie adjacent to another" .)
'be next to' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for abut that I've seen before include "Touch, lean on" , "Be next door to" , "Cease (from activity)" , "Touch (side of next building)" , "Meet" .)