Agreement plane needs to be shortened (7)

I believe the answer is:

concord

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'agreement' is the definition.
(concord is a kind of agreement)

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

(Other definitions for concord that I've seen before include "Amity" , "Harmony; treaty" , "Harmonious state of things" , "Agreement, harmony" .)

I've seen this clue in The Times.

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