March on a reservation (4)
I believe the answer is:
abut
'march' is the definition.
(both can mean to border or be adjacent to)
'on a reservation' is the wordplay.
'on' says to put letters next to each other.
'reservation' becomes 'but' (a 'but' is a reservation or objection to something**).
'a'+'but' is 'ABUT'.
(Other definitions for abut that I've seen before include "Adjoin, touch" , "Of building or land, end or lean against" , "Adjoin, border" , "Touch (side of next building)" , "Adjoin -- 19 (rev)" .)