The most common and most likely answer to this crossword clue is the 7 letter word WHITING
I've seen this crossword clue in The Times
W
H
I
T
I
N
G
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 = whiting.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 WHITING mean?
WHITING verb- To make white; to whiten; to bleach.
- A fine white chalk used in paints, putty, whitewash etc.
- A fish, Merlangius merlangus, similar to cod, found in the North Atlantic; English whiting (US).
- Any fish of many species that resemble Merlangius merlangus: