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