In the market for square pirate ship with no third sail (6)
I believe the answer is:
trader
'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" .)