First one drink's taken in by mouth (10)
I believe the answer is:
originally
'first' is the definition.
(I've seen this before)
'one drink's taken in by mouth' is the wordplay.
'one' becomes 'i' (Roman numeral).
'drink' becomes 'gin' (gin is a kind of drink).
'taken in by' indicates putting letters inside.
'mouth' becomes 'orally' (I've seen this in other clues).
'i'+'gin'='igin'
'igin' inserted into 'orally' is 'ORIGINALLY'.
(Other definitions for originally that I've seen before include "At the start" , "new way" , "In the first place" , "to begin with" .)