There Once Was a Cop(1972)

Campana, a commissar at the drugs bureau of Paris police gets an undercover job in Nice to catch an Italian-French mafia boss. As he slips into the identity of a gangster's brother, who was already killed, he has to play the role of a loving husband with child - something that is not so easy for a confirmed single like Campana. Unfortunately American killers chase behind the Nice mafia clan too, in order to get control of French drug trades to USA.