Protection for postmen? (4)

I believe the answer is:

mail

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'protection' is the definition.
(mail is a kind of protection)

I don't understand the rest of the clue.

(Other definitions for mail that I've seen before include "Armour of interlinked metal rings" , "some self-defence material" , "Post in America" , "American post from Lima" , "Post or body armour" .)

I've seen this clue in The Irish News.

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