The Collaborative International Dictionary of English v.0.48:

Expositive \Ex*pos"i*tive\, a.
   Serving to explain; expository. --Bp. Pearson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

expositive
    adj 1: serving to expound or set forth; "clean expository
           writing" [syn: expository, expositive]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "expositive":
   allegorizing, annotative, clarifying, commentative, commentatorial,
   critical, delineative, demonstrative, demythologizing, depictive,
   descriptive, discoursive, disquisitional, dissertational,
   editorial, elucidative, enlightening, essayistic, euhemeristic,
   exegetic, exemplificative, explaining, explanatory, explicative,
   explicatory, expositional, expositorial, expository, expressive,
   faithful, glossarial, graphic, illuminating, illuminative,
   illustrative, interpretive, lifelike, monographic, naturalistic,
   rationalistic, rationalizing, realistic, representative,
   scholiastic, true to life, vivid, well-drawn