The French blunder? Go wild (3,3)
I believe the answer is:
let rip
'go wild' is the definition.
(I've seen this before)
'the french blunder?' is the wordplay.
'the french' becomes 'le' ('the' in French).
'blunder?' becomes 'trip' (trip is a kind of blunder).
'le'+'trip'='LET RIP'
(Other definitions for let rip that I've seen before include "Release pent-up feelings" , "Hasten" , "Act violently" , "Sound off" , "Speak violently - allow to tear" .)