For a drink, agreed on a cocktail (9)
I believe the answer is:
orangeade
'for a drink' is the definition.
(orangeade is a kind of drink)
'agreed on a cocktail' is the wordplay.
'cocktail' indicates an anagram.
'agreed'+'on'+'a'='agreedona'
'agreedona' with letters rearranged gives 'ORANGEADE'.
(Other definitions for orangeade that I've seen before include "Squash" , "Fruit drink" , "Fruity drink" , "Bubbly drink" , "Soft citrus drink" .)