The French on excursion act without restraint (3,3)
I believe the answer is:
let rip
'act without restraint' is the definition.
(I've seen this before)
'the french on excursion' is the wordplay.
'the french' becomes 'le'.
'on' is a charade indicator (letters next to each other).
'excursion' becomes 'trip' (I've seen this before).
'le'+'trip'='LET RIP'
(Other definitions for let rip that I've seen before include "Act violently" , "Release pent-up feelings" , "Triple (anag)" , "Sound off" , "Act unrestrainedly" .)