Bird has one drink (7)
I believe the answer is:
martini
'drink' is the definition.
(I know that Martini is a Vermouth)
'bird has one' is the wordplay.
'bird' becomes 'martin' (martin is a kind of bird).
'has' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'martin'+'i'='MARTINI'
(Other definitions for martini that I've seen before include "Sienese painter" , "alcoholic drink" , "tipple" , "Italian drink" , "Alternative name for Simone Memmi, pre-Renaissance Italian painter" .)