Sweet that's doubly good in France (6)
I believe the answer is:
bonbon
'sweet' is the definition.
(bonbon is a kind of sweet)
'doubly good in france' is the wordplay.
'doubly' indicates the doubling of some letters.
'good in france' becomes 'bon' ('good' in French).
'bon' duplicated is 'BONBON'.
'that's' is the 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" .)