What does the cryptic crossword clue "A queen on board in swimming costume? (3-5)" mean?

The wordplay uses a charade (building blocks). ONE (synonym="ONE") + PIECE (synonym="PIECE") = ONEPIECE The answer is ONEPIECE.

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

Guardian Cryptic #29240 | 18 Down | 2023-11-29 flower