fifth column

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A clandestine subversive organization working within a country to further an invading enemy's military and political aims.

Wiktionary

1. n. dated A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal.

GNU Webster's 1913

1. n. a group of persons inside the battle lines of a territory engaged in a conflict, who secretly sympathize with the enemy, and who engage in espionage or sabotage; -- sometimes also referred to as a trojan horse.
2. n. Hence, any faction of persons within a group who secretly sympathize with an enemy, especially those who engage in activities harmful to the group; an enemy in one's midst; a group of traitors.

WordNet 3.0

1. n. a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst