Fabulous group fashionably late appearing in ancient city (3,7)

'fabulous group' is the definition.
(The Beatles were called the Fab Four)

'fashionably late appearing in ancient city' is the wordplay.
'fashionably' indicates an anagram (letters remade or refashioned).
'appearing in' means one lot of letters goes inside another.
'ancient city' becomes 'thebes' (ancient Greek city).
'late' is an anagram of 'eatl'.
'eatl' inserted inside 'thebes' is 'THE BEATLES'.