Join with one reservation (4)
I believe the answer is:
abut
'join' is the definition.
(abut can mean to join or be adjacent to)
'with one reservation' is the wordplay.
'with' is a charade indicator (letters next to each other).
'one' becomes 'a' (a thing is one thing).
'reservation' becomes 'but' (a 'but' is a reservation or objection to something).
'a' next to 'but' is 'ABUT'.
(Other definitions for abut that I've seen before include "Be next to -- share a boundary" , "Adjoin, be contiguous to" , "Lie adjacent to" , "Touch (side of next building)" , "Lie next to" .)