Guy that mixed a new cocktail (9)
I believe the answer is:
manhattan
'cocktail' is the definition.
(manhattan is a kind of cocktail)
'guy that mixed a new' is the wordplay.
'guy' becomes 'man' (I've seen this before).
'mixed' is an anagram indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'that' anagrammed gives 'hatt'.
'man'+'hatt'+'a'+'n'='MANHATTAN'
(Other definitions for manhattan that I've seen before include "One of the five boroughs of New York City" , "Vermouth and whisky cocktail - New York borough" , "Vermouth and whiskey, with a dash of bitters" , "area of New York City" , "Cocktail consisting of whisky, vermouth and bitters" .)