Saint taking one drink (7)
I believe the answer is:
martini
'drink' is the definition.
(I know that Martini is a IBA official cocktail)
'saint taking one' is the wordplay.
'saint' becomes 'martin' (I am not sure about this - if you are sure you should believe this answer much more).
'taking' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'martin'+'i'='MARTINI'
Can you help me to learn more?
(Other definitions for martini that I've seen before include "Sienese painter" , "Type of Italian vermouth" , "alcoholic drink" , "Alternative name for Simone Memmi, pre-Renaissance Italian painter" , "obtained from a bottle" .)