Definitive Indian drink, in small volume (7)
I believe the answer is:
classic
'definitive' is the definition.
(thesaurus)
'indian drink in small volume' is the wordplay.
'indian drink' becomes 'lassi' (I've seen this before).
'in' means one lot of letters goes inside another.
'small volume' becomes 'cc' (cubic centimetre).
'lassi' going into 'cc' is 'CLASSIC'.
(Other definitions for classic that I've seen before include "Judged over a long period to be of high quality" , "big race" , "work of the highest quality" , "Serving as a standard - of the highest quality" , "Very great work of art" .)