The most common and most likely answer to this crossword clue is the 12 letter word BEST-SELLING
I've seen this crossword clue in The Times
B
E
S
T
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S
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L
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I
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G
More clues leading to the result BEST-SELLING
Buddy explains!
The straight part
We know that "Very popular" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very popular = best-selling.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.
What does BEST-SELLING mean?
BEST-SELLING adjective- Highly popular with consumers purchasing goods within a specific class.