Cryptic crossword clue
Book about fish keeps being a best-seller 3,4

The most common and most likely answer to this crossword clue is the 3 plus 4 letter word HOT CAKE

I've seen this crossword clue in Independent
H
O
T
C
A
K
E

Buddy explains!

The straight part
We know that "a best-seller" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a best-seller = hot cake.
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.

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