Overthrow of French achievement (6)
I believe the answer is:
defeat
'overthrow' is the definition.
(I've seen this before)
'of french achievement' is the wordplay.
'of french' becomes 'de' ('of' in French).
'achievement' becomes 'feat' (feat is a kind of achievement).
'de'+'feat'='DEFEAT'
(Other definitions for defeat that I've seen before include "Beat (someone)" , "best or worst" , "Win victory over" , "Get the better of" , "Beat, win against" .)