Sweet that's doubly good in France (6)
I believe the answer is:
bonbon
'sweet' is the definition.
(I know that bonbon is a type of sweet)
'doubly good in france' is the wordplay.
'doubly' says to double the letters.
'good in france' becomes 'bon' (French for 'good').
'bon' duplicated is 'BONBON'.
'that's' acts as a link.
(Other definitions for bonbon that I've seen before include "a tasty sweet - in France" , "French sweet" , "Sweetie" , "Candy, doubly good in France?" , "Confection" .)