What's drunk agreed on, imbibing a soft drink (9)
I believe the answer is:
orangeade
'soft drink' is the definition.
(I've seen this before)
'what's drunk agreed on imbibing a' is the wordplay.
'what's drunk' indicates anagramming the letters.
'imbibing' is an insertion indicator.
'agreed'+'on'='agreedon'
'agreedon' with letters rearranged gives 'orangede'.
'orangede' enclosing 'a' is 'ORANGEADE'.
(Other definitions for orangeade that I've seen before include "Fruity drink" , "Fruit drink" , "Squash" , "Bubbly drink" , "Soft citrus drink" .)