Swallow iodine in drink (7)
I believe the answer is:
martini
'drink' is the definition.
(martini is a kind of drink)
'swallow iodine' is the wordplay.
'swallow' becomes 'martin' (martin is a kind of swallow).
'iodine' becomes 'I' (chemical symbol).
'martin'+'i'='MARTINI'
'in' is the link.
(Other definitions for martini that I've seen before include "Italian drink" , "Cocktail of gin and dry vermouth" , "tipple" , "Gin or vodka with dry vermouth" , "alcoholic drink" .)