The most common and most likely answer to this crossword clue is the 7 plus 3 plus 5 letter word SINGING THE BLUES
I've seen this crossword clue in The Guardian
S
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N
G
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T
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B
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Buddy explains!
The straight part
We know that "once popular number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case once popular number = singing the blues.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.