Swallow single cocktail (7)
I believe the answer is:
martini
'cocktail' is the definition.
(martini is a kind of cocktail)
'swallow single' is the wordplay.
'swallow' becomes 'martin' (martin is a kind of swallow).
'single' becomes 'i' (one**).
'martin'+'i'='MARTINI'
(Other definitions for martini that I've seen before include "tipple" , "Cocktail made with gin or vodka and vermouth" , "In glass, one's dry" , "Alternative name for Simone Memmi, pre-Renaissance Italian painter" , "Cocktail of gin and dry vermouth" .)