Swallow one cocktail (7)
I believe the answer is:
martini
'cocktail' is the definition.
(martini is a kind of cocktail)
'swallow one' is the wordplay.
'swallow' becomes 'martin' (martin is a kind of swallow).
'one' becomes 'i' (Roman numeral).
'martin'+'i'='MARTINI'
(Other definitions for martini that I've seen before include "tipple" , "obtained from a bottle" , "Gin or vodka with dry vermouth" , "Cocktail of gin and dry vermouth" , "alcoholic drink" .)