A lesson is unethical (6)
I believe the answer is:
amoral
'unethical' is the definition.
(I've seen this before)
'a lesson' is the wordplay.
'lesson' becomes 'moral' (as in the moral of the story).
'a'+'moral'='AMORAL'
'is' acts as a link.
(Other definitions for amoral that I've seen before include "Having no standards of right or wrong" , "Unconcerned with correctness" , "unconcerned about ethics" , "Unprincipled" , "outrageous!" .)