Crossword Genius

A skinny penalty! (4)

Ross

I believe the answer is:

fine

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'penalty' is the definition.
(a financial penalty)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for fine that I've seen before include "Sheer or very good" , "Magnificent" , "Very thin; punishment" , "end to the music" , "nice > brandy" .)

I've seen this clue in the Sydney Morning Herald.
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