The most common and most likely answer to this crossword clue is the 8 letter word EELPOUTS
I've seen this crossword clue in The Times
E
E
L
P
O
U
T
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 = eelpouts.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 EELPOUTS mean?
EELPOUTS noun- Any fish of the family Zoarcidae.
- A yellow flower of uncertain type, possibly the eel-ware, Ranunculus fluitans.