Making a grade one drink (9)
I believe the answer is:
orangeade
'drink' is the definition.
(orangeade is a kind of drink)
'making a grade one' is the wordplay.
'making' indicates anagramming the letters.
'a'+'grade'+'one'='agradeone'
'agradeone' with letters rearranged gives 'ORANGEADE'.
(Other definitions for orangeade that I've seen before include "Squash" , "Fruity drink" , "Bubbly drink" , "Soft citrus drink" , "Pop" .)