The Collaborative International Dictionary of English v.0.48:

columnist \col"um*nist\, n.
   a journalist who writes or edits a regularly scheduled
   column[8] in a periodical, usually in editorial style; a type
   of editorialist.
   [RP]

   fifth columnist see fifth columnist in the vocabulary.

WordNet (r) 3.0 (2006):

fifth columnist
    n 1: a member of a clandestine subversive organization who tries
         to help a potential invader [syn: fifth columnist,
         saboteur]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "fifth columnist":
   Trojan horse, apostate, backslider, bolter, collaborationist,
   collaborator, convert, crypto, defector, deserter, fellow traveler,
   fifth column, fraternizer, mugwump, proselyte, quisling,
   recidivist, recreant, renegade, renegado, renegate, reversionist,
   runagate, saboteur, schismatic, seceder, secessionist,
   security risk, separatist, strikebreaker, subversive, tergiversant,
   tergiversator, traitor, turnabout, turncoat, turntail,
   underground