Servant that developed a new cocktail (9)
I believe the answer is:
manhattan
'cocktail' is the definition.
(I know that Manhattan is a type of cocktail)
'servant that developed a new' is the wordplay.
'servant' becomes 'man' (man is a kind of servant).
'developed' indicates an anagram.
'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 "Island of New York - cocktail" , "Vermouth and whisky cocktail - New York borough" , "Cocktail consisting of whisky, vermouth and bitters" , "Vermouth and whiskey, with a dash of bitters" , "New York City borough" .)