The most common and most likely answer to this crossword clue is the 5 letter word MORAY
M
O
R
A
Y
More clues leading to the result MORAY
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 = moray.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 MORAY mean?
MORAY noun- Any of the large cosmopolitan carnivorous eels of the family Muraenidae.