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]