Crossword Genius

Chaffinch's tailpiece? (3)

Ross

I believe the answer is:

fin

I'm a little stuck... Click here to teach me more about this clue!

'tailpiece?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?

'chaffinch's' is the wordplay.
'FIN' is hidden within the letters.
But, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for fin that I've seen before include "Part of an aircraft's tail" , "Swimmer's flipper" , "A fish uses it to steer itself" , "Paddle-like organ" , "For movement and balance in fish" .)

I've seen this clue in the Evening Standard.
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