The most common and most likely answer to this crossword clue is the 4 letter word OPAH
I've seen this crossword clue in Financial Times
O
P
A
H
More clues leading to the result OPAH
- Fish over on Maori settlement
- Fish – a shop serves more than one, battered
- Big fish sending Marx up heartlessly?
- US TV celebrity losing heart for fish
- Big fish creating no splash odd characters missed
- Fish stocked by shop, a halibut
- Fish in shop, a halibut
- Work on a hard fish
- Stop a hoodlum from smuggling moonfish
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 = opah.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 OPAH mean?
OPAH noun- Any of various large, colourful, deep-bodied pelagic fish of the family Lamprididae.