The most common and most likely answer to this crossword clue is the 7 letter word LOACHES
I've seen this crossword clue in Financial Times
L
O
A
C
H
E
S
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 = loaches.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 LOACHES mean?
LOACHES noun- Acronym of light observation helicopter.
- A bottom-feeding freshwater fish in the superfamily or suborder Cobitoidea.