Fabulous quartet without rhythm? Not entirely (7)
I believe the answer is:
beatles
'fabulous quartet without' is the definition.
I can't tell whether this defines the answer.
'rhythm? not entirely' is the wordplay.
'rhythm?' becomes 'beat' ('beat' can be a synonym of 'rhythm').
'not entirely' becomes 'les' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'beat'+'les'='BEATLES'
Can you help me to learn more?
(Other definitions for beatles that I've seen before include "Group of British" , "Best ale for the Liverpudlian group" , "John, Paul, Ringo and George - collectively" , "1960s' Liverpool rock group" , "British pop band of the 60s" .)