A skinny penalty! (4)
I believe the answer is:
fine
'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" .)