Legendary like the Beatles were first (6)
I believe the answer is:
fabled
'legendary' is the definition.
(synonyms)
'like the beatles were first' is the wordplay.
'like the beatles' becomes 'fab' (the Beatles were called the Fab Four).
'were first' becomes 'led' (if people led, they were first or went ahead).
'fab'+'led'='FABLED'
(Other definitions for fabled that I've seen before include "Legendary, mythical" , "noted" , "Well-known" , "Famous; imaginary" , "Told of in legends or myths" .)