Crossword Genius

In the market for square pirate ship with no third sail (6)

Ross

I believe the answer is:

trader

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'in' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for trader that I've seen before include "One in business" , "Tarred like merchant" , "One selling" , "Dealer who buys and sells" , "He buys and sells goods" .)

I've seen this clue in The Irish Times.
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