Chap that mixed a new cocktail (9)
I believe the answer is:
manhattan
'cocktail' is the definition.
(manhattan is a kind of cocktail)
'chap that mixed a new' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'mixed' is an anagram indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'that' with letters rearranged gives 'hatt'.
'man'+'hatt'+'a'+'n'='MANHATTAN'
(Other definitions for manhattan that I've seen before include "Vermouth and whiskey, with a dash of bitters" , "It blended with spirit" , "intoxicating drink" , "Island of New York - cocktail" , "One of the five boroughs of New York City" .)