Crossword Genius

A fishy ending (3)

Ross

I believe the answer is:

fin

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'a fishy ending' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

This is the entire clue.

Can you help me to learn more?

(Other definitions for fin that I've seen before include "For movement and balance in fish" , "Fishy appendage" , "Piscine appendage" , "Fish propeller; end (Fr.)" , "A fish uses it to steer itself" .)

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