Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

undeceive
     v : free from deception or illusion [ant: {deceive}]