The most common and most likely answer to this crossword clue is the 6 letter word BONITO
B
O
N
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T
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More clues leading to the result BONITO
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 = bonito.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 BONITO mean?
BONITO noun- Any of various marine fish of the genus Sarda, that are related to and resemble the tuna.
- A large tropical fish, the skipjack tuna, allied to the tunny, Katsuwonus pelamis.
- The medregal (Seriola fasciata), an edible fish of the southern of the United States and the West Indies.
- The cobia or crab eater (Rachycentron canadum), an edible fish of warm waters globally.