undeceive

from WordNet (r) 3.0 (2006)

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

from 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]