The most common and most likely answer to this crossword clue is the 3 letter word GAR
I've seen this crossword clue in The Times
G
A
R
More clues leading to the result GAR
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 = gar.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 GAR mean?
GAR noun- A spear.
- Any of several North American fish of the family Lepisosteidae that have long, narrow jaws.
- A garfish, Belone belone.
- To make, compel (someone to do something); to cause (something to be done).