A grain's used to make a drink (7)
I believe the answer is:
sangria
'a drink' is the definition.
(type of alcoholic drink)
'a grain's used' is the wordplay.
'used' indicates anagramming the letters.
'a'+'grains'='agrains'
'agrains' is an anagram of 'SANGRIA'.
'to make' is the link.
(Other definitions for sangria that I've seen before include "Mixed drink" , "Spicy Spanish wine drink" , "Spanish spiced wine" , "Spanish drink of red wine with soda water or lemonade" , "Spanish wine cocktail" .)