A drink British always mature (8)
I believe the answer is:
beverage
'a drink' is the definition.
('beverage' can be a synonym of 'drink')
'british always mature' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'mature' becomes 'age' ('age' can be a synonym of 'mature').
'b'+'ever'+'age'='BEVERAGE'
(Other definitions for beverage that I've seen before include "Any drink, say a beer" , "Prepared drink" , "Liquid for drinking" , "such as a Frappuccino" , "Tea, say" .)