American that's used by Nato to finish affray (6)

I believe the answer is:

yankee

I'm a little stuck... Click here to teach me more about this clue!

'american' is the definition.
(I know that Yankee is a type of american)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for yankee that I've seen before include "US northerner to a southerner" , "A bet; US Federal soldier" , "A bet on four or more horses in different races" , "Northern US inhabitant" , "A bet on several horses at once - US citizen" .)

I've seen this clue in The Guardian.

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