The most common and most likely answer to this crossword clue is the 6 letter word BARBEL
I've seen this crossword clue in The Guardian
B
A
R
B
E
L
More clues leading to the result BARBEL
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 = barbel.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 BARBEL mean?
BARBEL noun- A freshwater fish of the genus Barbus or other closely related genera.
- Whisker-like sensory organs, located around the mouth of certain fish, including catfish, carp, goatfish, sturgeon, and some types of shark.
- A barb or pap under the tongues of horses and cattle.