Countries may share it between them (8)
I believe the answer is:
frontier
'countries may' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for frontier that I've seen before include "Wilderness at the edge of a settled area" , "Land-border of a country" , "Limit" , "Land border of a country" , "wild area" .)