Neighbor of Montana (5)
I believe the answer is:
idaho
'neighbor of montana' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for idaho that I've seen before include "The Gem State, known for its potatoes" , "NW state of USA" , "State of NW US" , "My Own Private -- (1991 film)" , "US state S. of Montana" .)