The Collaborative International Dictionary of English v.0.48:

Expositor \Ex*pos"i*tor\, n. [L. See Expound.]
   One who, or that which, expounds or explains; an expounder; a
   commentator. --Bp. Horsley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

expositor
    n 1: a person who explains [syn: expositor, expounder]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "expositor":
   Boanerges, allegorist, annotator, author, book reviewer,
   chalk talker, cicerone, clarifier, commentator, commenter, critic,
   cryptanalyst, cryptographer, cryptologist, decoder, definer,
   demonstrator, demythologizer, descanter, diaskeuast, discourser,
   disquisitor, dragoman, editor, editorial writer, editorialist,
   emendator, emender, essayist, euhemerist, exegesist, exegete,
   exegetist, explainer, explicator, exponent, expounder, glossarist,
   glossographer, go-between, guide, hermeneut, homilist, interpreter,
   leader writer, lecturer, lexicographer, metaphrast, monographer,
   monographist, news analyst, oneirocritic, paraphrast, praelector,
   preacher, publicist, pulpitarian, pulpiteer, reader, reviewer,
   scholiast, sermoner, sermonist, sermonizer, textual critic,
   tractation, translator, writer