Comely youth, one entering quarters after a party (6)
I believe the answer is:
adonis
'comely youth' is the definition.
'one entering quarters after a party' is the wordplay.
'one entering quarters' becomes 'nis' (I can't justify this - if you can you should believe this answer much more).
'after' is a charade indicator (letters next to each other).
'party' becomes 'do' (do is a kind of party).
'a'+'do'='ado'
'nis' after 'ado' is 'ADONIS'.
Can you help me to learn more?
(Other definitions for adonis that I've seen before include "Handsome Greek loved by Aphrodite" , "Aphrodite's beloved" , "Handsome Gk. youth" , "Hunk" , "Handsome fellow" .)