A queen on board in swimming costume? (3-5)

I believe the answer is:

one-piece

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

'costume?' is the definition.
(I know that one piece is a type of costume)

'a queen on board in swimming' is the wordplay.
'a' becomes 'one'.
'queen on board in swimming' becomes 'piece' (I can't justify this - if you can you should give a lot more credence to this answer).
'one'+'piece'='ONE-PIECE'

Can you help me to learn more?

(Other definitions for one-piece that I've seen before include "Made as a single unit" , "Bathing-costume" , "(E.g. swimming costume) not as separates" , "Made as a single item" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.