Firm accessing a large population provides drink (7)
I believe the answer is:
alcopop
'drink' is the definition.
(type of alcoholic drink)
'firm accessing a large population' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'accessing' means one lot of letters goes inside another (inserted letters gain access).
'large' becomes 'L' (eg in clothes sizes).
'population' becomes 'pop' (I can't explain this - if you can you should believe this answer much more).
'a'+'l'+'pop'='alpop'
'co' inserted within 'alpop' is 'ALCOPOP'.
'provides' is the link.
Can you help me to learn more?
(Other definitions for alcopop that I've seen before include "fizzy drink" , "It tastes like a soft drink, but isn't!" , "drink for young adult" , "Sweet pub drink" , "booze for teenagers" .)