The most common and most likely answer to this crossword clue is the 5 letter word GUPPY
I've seen this crossword clue in The Times
G
U
P
P
Y
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 = guppy.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 GUPPY mean?
GUPPY noun- A tiny freshwater fish, Poecilia reticulata, popular in home aquariums, that usually has a plain body and black or dark blue tail for the females and a more colorful tail for the males.
- By extension, any tiny fish.
- A tube holding paintballs before they are loaded into the gun.