Multiply journey by the French (6)
I believe the answer is:
triple
'multiply' is the definition.
(I know that triple is a more specific form of the action multiply)
'journey by the french' is the wordplay.
'journey' becomes 'trip' (trip is a kind of journey).
'by' is a charade indicator (letters next to each other).
'the french' becomes 'le' ('the' in French).
'trip'+'le'='TRIPLE'
(Other definitions for triple that I've seen before include "Consisting of three" , "involving several parties" , "Threefold, treble" , "make score sixty?" , "Multiply by three" .)