a flash in the pan
US /ə flæʃ ɪn ðə pæn/
UK /ə flæʃ ɪn ðə pæn/
1.
a sudden but brief success or a person who achieves it
:
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Many people thought his first hit song was just a flash in the pan, but he went on to have a successful career.
•
We need consistent performance, not just a flash in the pan.