Delight over line adopted by youngster in choir (4,4)
I believe the answer is:
glee club
'choir' is the definition.
(I've seen this before)
'delight over line adopted by youngster' is the wordplay.
'delight' becomes 'glee' (glee can mean great joy).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'line' becomes 'l' (used when specifying particular lines from a poem).
'adopted by' is an insertion indicator (I've seen this in other clues).
'youngster' becomes 'cub' (young bear).
'l' inserted into 'cub' is 'club'.
'glee'+'club'='GLEE CLUB'
'in' acts as a link.
(Other definitions for glee club that I've seen before include "Group of singers" , "Choir" , "Group singing part-songs" , "Society for singing part songs" .)