The most common and most likely answer to this crossword clue is the 6 letter word TARPON
I've seen this crossword clue in The Times
T
A
R
P
O
N
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 = tarpon.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 TARPON mean?
TARPON noun- Any of several fishes of the family Elopidae or Megalopidae, especially a large silvery game fish.