Drink cola mixed by old man (7)
I believe the answer is:
alcopop
'drink' is the definition.
(type of alcoholic drink)
'cola mixed by old man' is the wordplay.
'mixed' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'old man' becomes 'pop' (both can mean one's father).
'cola' anagrammed gives 'alco'.
'alco'+'pop'='ALCOPOP'
(Other definitions for alcopop that I've seen before include "Smooth out" , "Intoxicating drink resembling a soft drink" , "Sweet pub drink" , "Hard drink resembling a soft drink" , "drink for young adult" .)