Chain letters? (4)

I think it could be either:

Which length description are you looking for?:

mail

I believe this clue is a double definition.

'chain' is the first definition.
(I've seen this before)

'letters' is the second definition.
(I've seen this in another clue)

(Other definitions for mail that I've seen before include "Letters - armour" , "Eg, letters" , "Armour; posted items" , "Paper" , "Form of armour; post" .)

I've seen this clue in The Sun, the Evening Standard and The Globe and Mail.

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