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"flatfish" is also used as a crossword clue. If that's what you saw, the answer is often brill (5 letters).
Definitions
A fish with a flattened body that swims on one side near the sea floor.
Word origin
From flat + fish.
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Side-swimming catch”8 letters
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