Countries may share them between them (9)

I believe the answer is:

frontiers

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'countries' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for frontiers that I've seen is " Countries' borders".)

I've seen this clue in The Irish News.

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