Crossword Genius

On ship overseas, Rex travels east (6)

Ross

I believe the answer is:

aboard

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'on ship' is the definition.
(aboard a ship)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for aboard that I've seen before include "on a ferry?" , "On ship or plane - not on land" , "On vessel or vehicle" , "on a train, perhaps" , "Ready to sail" .)

I've seen this clue in The Independent.
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