Idiom refers to a word, phrase or an expression whose collective meaning differs from their individual (words) meaning.
Flash in the pan
Meaning: Short-lived success; promises much, but fails to deliver.
Ex.: Tom got an A+ grade, which was not just a flash in the pan.
Finding your feet
Meaning: Becoming familiar with a new situation, gaining confidence in a new scenario.
Ex.: Paul is yet to find his feet in the entertainment industry.