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