The most common and most likely answer to this crossword clue is the 9 letter word ALBACORES
I've seen this crossword clue in Financial Times
A
L
B
A
C
O
R
E
S
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The straight part
We know that "Fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fish = albacores.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 ALBACORES mean?
ALBACORES noun- A large marine fish Thunnus alalunga of warm seas, having edible flesh.