A lesson is corrupt (6)
I believe the answer is:
amoral
'is corrupt' is the definition.
(thesaurus)
'a lesson' is the wordplay.
'lesson' becomes 'moral' (as in the moral of the story).
'a'+'moral'='AMORAL'
(Other definitions for amoral that I've seen before include "Ignoring what's right and wrong" , "Unconcerned with correctness" , "Having no standards of right or wrong" , "without guiding principles" , "Unconcerned about what's right or wrong" .)