The most common and most likely answer to this crossword clue is the 6 plus 1 letter word TROUTS -
T
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T
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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 = trouts -.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 TROUTS - mean?
TROUTS - noun- Any of several species of fish in Salmonidae, closely related to salmon, and distinguished by spawning more than once.
- An objectionable elderly woman.
- To (figuratively) slap someone with a slimy, stinky, wet trout; to admonish jocularly.