A large supermarket stores soft drink (7)
I believe the answer is:
alcopop
'drink' is the definition.
(type of alcoholic drink)
'a large supermarket stores soft' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'supermarket' becomes 'Co-op' (The Co-operative).
'stores' indicates putting letters inside.
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'coop' enclosing 'p' is 'copop'.
'a'+'l'+'copop'='ALCOPOP'
(Other definitions for alcopop that I've seen before include "drink young people like" , "which is sweetly intoxicating" , "Smooth out" , "Drink for young adults" , "It tastes like a soft drink, but isn't!" .)