fifth columnist

English Thesaurus

1. a member of a clandestine subversive organization who tries to help a potential invader (noun.person)

hypernym	:	traitor, treasonist,
definition	:	someone who betrays his country by committing treason (noun.person)

member meronym	:	fifth column, trojan horse,
definition	:	a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst (noun.group)