Gin cocktail? Swallow one! (7)
I believe the answer is:
'gin cocktail?' is the definition.
(I've seen this before)
'swallow one' is the wordplay.
'swallow' becomes 'martin' (martin is a kind of swallow).
'one' becomes 'i' (Roman numeral).
(Other definitions for martini that I've seen before include "Gin or vodka with dry vermouth" , "tipple" , "In glass, one's dry" , "rifle" , "James Bond's drink of choice" .)