Best in The Beatles, once? (7)
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" .)