Crossword Genius

A pianist of importance? (3,3)

Ross

I believe the answer is:

key man

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

'importance?' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'a pianist' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is present in the answer.
This explanation may well be incorrect...

'of' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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