Most sugary dessert is French (8)
I believe the answer is:
sweetest
'most sugary' is the definition.
('sweet' is associated in meaning with 'sugary')
'dessert is french' is the wordplay.
'dessert' becomes 'sweet' (synonyms).
'is french' becomes 'est' ('is' in French).
'sweet'+'est'='SWEETEST'
(Other definitions for sweetest that I've seen before include "'''Success is counted ...by those who ne'er succeed'' (Emily Dickinson) (8)'" , "Most winsome" , "dearest" , "Most saccharine or dulcet" , "Superlative winning" .)