Best of French achievement (6)
I believe the answer is:
defeat
'best' is the definition.
(thesaurus)
'of french achievement' is the wordplay.
'of french' becomes 'de'.
'achievement' becomes 'feat' (feat is a kind of achievement**).
'de'+'feat'='DEFEAT'
(Other definitions for defeat that I've seen before include "Trounce, beat" , "Loss" , "Rout" , "Opposite of victory" , "Overthrow" .)