The most common and most likely answer to this crossword clue is the 3 letter word IDE
I've seen this crossword clue in The Times
I
D
E
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 = ide.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 IDE mean?
IDE noun- A freshwater fish of the family Cyprinidae, found across northern Europe and Asia, especially Leuciscus idus.