What does the cryptic crossword clue "Stone in middle knocked out of game (4)" mean?

The definition is "game". The wordplay uses a deletion. RU (synonym="RU") + BY (synonym="game") = RUBY; definition: "Stone" The answer is RUBY.

Stone in middle knocked out of game (4)

Guardian Cryptic #29397 | 13 Across | 2024-05-31 flower