Can describe mail or just one letter (5)

I believe the answer is:

chain

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'letter' is the definition.
(chain letter is a kind of letter)

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

(Other definitions for chain that I've seen before include "Line of links" , "Paper decoration" , "Length of pitch" , "Range of mountains" , "Mountain range" .)

I've seen this clue in the Evening Standard.

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