Frenchman wearing waistcoat gets cocktail (6)
I believe the answer is:
gimlet
'cocktail' is the definition.
(gimlet is a kind of cocktail)
'frenchman wearing waistcoat' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'wearing' indicates putting letters inside.
'waistcoat' becomes 'gilet' (I've seen this before).
'm' going inside 'gilet' is 'GIMLET'.
'gets' acts as a link.
(Other definitions for gimlet that I've seen before include "Small hand tool - cocktail of vodka or gin with lime juice" , "One pierces" , "a boring thing" , "The Netherlands - are islands in the Caribbean" , "Tool for boring holes and a cocktail" .)