Idiom refers to a word, phrase or an expression whose collective meaning differs from their individual (words) meaning.
Turn over a new leaf
Meaning: To change for the better, try to improve, give up bad habits, etc.
Ex.: After his bullish behavior landed him in serious trouble with the cops, Tom turned over a new leaf.
Wild goose chase
Meaning: fruitless search or pursuit, unsuccessful search, etc.
Ex.: The robbers made the cops to go on a wild goose chase.