The most common and most likely answer to this crossword clue is the 8 letter word RAT-TAIL
I've seen this crossword clue in The Times
R
A
T
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T
A
I
L
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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 = rat-tail.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 RAT-TAIL mean?
RAT-TAIL noun- A towel that has been tightly twisted along the diagonal to make a rudimentary whip with a towel corner at the tip, typically used in juvenile pranks.
- An excrescence growing from the pastern to the middle of the shank of a horse.
- A fish, the California chimera, Hydrolagus colliei, syn. Chimaera colliei.
- Any fish of the genus Macrourus; a grenadier.