Crossword Genius

Top US sporting figure (6,3)

Ross

I believe the answer is:

salary cap

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

'us sporting figure' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'top' is the wordplay.
I cannot quite understand how this works, but
'top' could be 'cap' (both can mean to beat or outdo) and 'cap' is present in the answer.
The remaining letters 'salary' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Sunday Times.
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