Composer twice drinks litre before start of symphony (5)
I believe the answer is:
bliss
'composer' is the definition.
(British composer Arthur Bliss)
'twice drinks litre before start of symphony' is the wordplay.
'twice' becomes 'bis' (musical direction meaning to play a second time).
'drinks' indicates putting letters inside.
'litre' becomes 'L' (abbreviation).
'before' is a charade indicator (letters next to each other).
'start of' indicates taking the first letters.
The initial letter of 'symphony' is 's'.
'bis' placed around 'l' is 'blis'.
'blis'+'s'='BLISS'
(Other definitions for bliss that I've seen before include "One who wrote music" , "Great joy" , "Extreme happiness" , "rhapsody?" , "Serene joy" .)