The most common and most likely answer to this crossword clue is the 6 letter word SHARKS
I've seen this crossword clue in Financial Times
S
H
A
R
K
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 = sharks.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 SHARKS mean?
SHARKS noun- A scaleless, predatory fish of the superorder Selachimorpha, with a cartilaginous skeleton and 5 to 7 gill slits on each side of its head.
- To fish for sharks.
- Someone who exploits others, for example by trickery, lies, usury, extortion.
- A sleazy and amoral lawyer
- An ambulance chaser.
- A relentless and resolute person or group, especially in business.
- A very good poker or pool player. Compare fish (a bad poker player).
- A person who feigns ineptitude to win money from others.
- To steal or obtain through fraud.
- To play the petty thief; to practice fraud or trickery; to swindle.
- To live by shifts and stratagems.
- To pick or gather indiscriminately or covertly.