The Collaborative International Dictionary of English v.0.48:

Undeceive \Un`de*ceive"\, v. t. [1st pref. un- + deceive.]
   To cause to be no longer deceived; to free from deception,
   fraud, fallacy, or mistake. --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undeceive
    v 1: free from deception or illusion [ant: betray, deceive,
         lead astray]