Its main purpose was to convey information (4,4)

I believe the answer is:

mail boat

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'information' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for mail boat that I've seen is " Packet".)

I've seen this clue in The Globe and Mail.

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