The most common and most likely answer to this crossword clue is the 6 letter word TAUTOG
I've seen this crossword clue in The Guardian
T
A
U
T
O
G
More clues leading to the result TAUTOG
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 = tautog.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 TAUTOG mean?
TAUTOG noun- A fish of the wrasse family found in salt water off of eastern North America from Nova Scotia to South Carolina (Tautoga onitis).