“That’ll never ___”

The phrase “That’ll never fly” is a common idiomatic expression that means something is unlikely to succeed or be accepted. The word “fly” in this context refers to the idea of soaring or taking flight, like an airplane. The origin of the phrase is uncertain, but it may have originated from early aviation history when engineers and inventors were trying to design flying machines that were considered impossible or impractical. Nowadays, the phrase is used more broadly and can be applied to any situation where success seems unlikely or doubtful. In the crossword clue, the correct answer to the blank space is “fly” because it completes the idiomatic phrase.