To adapt tales is to plagiarise! (5)
I believe the answer is:
steal
'is to plagiarise' is the definition.
(I've seen this before)
'to adapt tales' is the wordplay.
'to adapt' indicates anagramming the letters (I've seen 'adapting' mean this).
'tales' anagrammed gives 'STEAL'.
(Other definitions for steal that I've seen before include "Take unlawfully" , "Pilfer" , "Rob" , "Purloin" , "Walk off with" .)