A secret agent who incites suspected persons to commit illegal acts WordNet 3.0 & Open English WordNet

An agent provocateur is a person who commits, or who acts to entice another person to commit, an illegal or rash act or falsely implicates them in partaking in an illegal act, so as to ruin the reputation of, or entice legal action against, the target, or a group they belong to or are perceived to belong to. Wikipedia

Person employed to act undercover to entice or provoke another person to commit an illegal or rash act. Wikipedia Disambiguation

A person who secretly disrupts a group's activities from within the group; an instigator, troublemaker. Wiktionary

A person who disrupts a group's activities from within. Wiktionary (translation)

An undercover agent who incites suspected persons to partake in or commit criminal acts. Wiktionary

Undercover agent who incites suspected persons to partake in or commit criminal acts. Wiktionary (translation)