The French on journey behave uncontrollably (3,3)
I believe the answer is:
let rip
'behave uncontrollably' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'the french on journey' is the wordplay.
'the french' becomes 'le' ('the' in French).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'journey' becomes 'trip' (trip is a kind of journey).
'le'+'trip'='LET RIP'
Can you help me to learn more?
(Other definitions for let rip that I've seen before include "Speak violently - allow to tear" , "Act unrestrainedly" , "Behave without restraint" , "Sound off" , "Express oneself without restraint" .)