The most common and most likely answer to this crossword clue is the 8 letter word FLATHEAD
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More clues leading to the result FLATHEAD
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 = flathead.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 FLATHEAD mean?
FLATHEAD noun- Any fish in the Platycephalidae family.
- (plural only "flatheads") A type of screw or bolt designed to fit in a countersink so that it sits flush with a surface.
- (plural only "flatheads") A type of engine that has the valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine.
- A large freshwater catfish native to North America, Pylodictis olivaris.