The most common and most likely answer to this crossword clue is the 5 letter word LINGS
I've seen this crossword clue in Financial Times
L
I
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G
S
Buddy explains!
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 = lings.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 LINGS mean?
LINGS noun- Any of various marine food fish, of the genus Molva, resembling the cod.
- The common ling, Molva molva.
- Any of various varieties of heather or broom.