Top tune? (6,3)

I think it could be either:

Which length description are you looking for?:

6,3

number one

(Other definitions for number one that I've seen before include "Informally the self, numerically speaking" , "Yours truly" , "top of the pops" , "it concerns egoists" , "Wee" .)

I've seen this clue in The Guardian.

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