Sweet drink in most of France once (6)
I believe the answer is:
gateau
'sweet' is the definition.
(type of dessert or sweet)
'drink in most of france once' is the wordplay.
'drink' becomes 'tea' (tea is a kind of drink).
'in' is an insertion indicator.
'most of' means to remove the last letter.
'france once' becomes 'Gaul' (historical region including modern-day France).
'gaul' with its final letter taken off is 'gau'.
'tea' placed within 'gau' is 'GATEAU'.
(Other definitions for gateau that I've seen before include "Rich dessert cake" , "Richly decorated cake" , "Cake for Pierre" , "Fancy cake" , "Black Forest dessert" .)