Crossword Genius

Postal packet that goes overseas (4,4)

Ross

I believe the answer is:

mail boat

I'm a little stuck... Click here to teach me more about this clue!

'postal packet' is the definition.
'mail boat' can be an answer for 'packet' (synonyms). I am not certain of the 'postal' bit.

I cannot understand how the rest 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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