Most outspoken diarist is French (8)
I believe the answer is:
frankest
'most outspoken' is the definition.
('frank' can be a synonym of 'outspoken')
'diarist is french' is the wordplay.
'diarist' becomes 'Frank' (Anne Frank).
'is french' becomes 'est' ('is' in French).
'frank'+'est'='FRANKEST'
(Other definitions for frankest that I've seen before include "Most open and candid" , "extremely open" .)