fifth-columnist       
noun [ C ] uk/ˌfɪfθ ˈkɒl.əm.nɪst/ us/ˌfɪfθ ˈkɑː.ləm.nɪst/
 第五纵队成员(支持并秘密帮助所在国家的敌人的一群人)
a member of a group of people who support the enemies of the country they live in and secretly help them
  He was ambivalent about his role as a fifth-columnist.  他对自己作为第五纵队成员的角色感到很矛盾。 
 The writer was secretly a fifth-columnist.  这位作家暗地里是第五纵队的成员。 
 See: fifth column
 - More examples
 - The political establishment was looking for a fifth-columnist to attack.
 - It takes only one fifth-columnist to pass on sensitive information.
 - This man is a treacherous fifth columnist.