The French going on journey behave without restraint (3,3)
I believe the answer is:
let rip
'behave without restraint' is the definition.
(I've seen this before)
'the french going on journey' is the wordplay.
'the french' becomes 'le' ('the' in French).
'going on' means one lot of letters go next to another.
'journey' becomes 'trip' (trip is a kind of journey).
'le'+'trip'='LET RIP'
(Other definitions for let rip that I've seen before include "Express oneself without restraint" , "Behave without restraint" , "Act violently" , "Go wild" , "Act without restraint" .)