Sweet leaves in France once cut (6)
I believe the answer is:
gateau
'sweet' is the definition.
(type of dessert or sweet)
'leaves in france once cut' is the wordplay.
'leaves' becomes 'tea' (as in tea leaves).
'in' is an insertion indicator.
'france once' becomes 'Gaul' (historical region including modern-day France).
'cut' means to remove the last letter.
'gaul' with its last letter removed is 'gau'.
'tea' put inside 'gau' is 'GATEAU'.
(Other definitions for gateau that I've seen before include "It's sweet" , "Black Forest dessert" , "Cake (from the Black Forest?)" , "(French?) cake" , "Fancy cake" .)