Crossword Genius

Best in The Beatles, once? (7)

Ross

I believe the answer is:

drummer

'best' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?

I don't understand how the rest of the clue works.

(Other definitions for drummer that I've seen before include "Band's percussionist" , "eg Buddy Rich" , "Group member" , "eg Ringo" , "dozen Christmas presents" .)

I've seen this clue in The Independent.
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