A large police mission finding drink (7)
I believe the answer is:
alcopop
'drink' is the definition.
(type of alcoholic drink)
'a large police mission' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'police' becomes 'cop' (cop can mean informally mean a police officer).
'mission' becomes 'op' (operation).
'a'+'l'+'cop'+'op'='ALCOPOP'
'finding' acts as a link.
(Other definitions for alcopop that I've seen before include "fizzy drink" , "Polo cap (anag.)" , "drink for young adult" , "Mixed drink" , "which is sweetly intoxicating" .)