Singer, a large youth overcome by drink (9)
I believe the answer is:
balladeer
'singer' is the definition.
(someone singing ballads)
'a large youth overcome by drink' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'youth' becomes 'lad' (lad means a young man).
'overcome by' means one lot of letters goes inside another.
'drink' becomes 'beer' (beer is a kind of drink).
'a'+'l'+'lad'='allad'
'allad' placed within 'beer' is 'BALLADEER'.
(Other definitions for balladeer that I've seen before include "One sings" , "One making a song" , "pop singer" , "Minstrel" .)