Stop saying “Six” in French (5)
I believe the answer is:
cease
'stop' is the definition.
('cease' can be a synonym of 'stop')
'saying six in french' is the wordplay.
'saying' indicates a 'sounds like' (homophone) clue.
'six in french' becomes 'six' ('six' in French).
'six' sounds like 'CEASE'.
(Other definitions for cease that I've seen before include "Culminate" , "Discontinue" , "Come to an end" , "Stop, give over" , "Political group" .)