Aesop's lesson (5)
I believe the answer is:
moral
'aesop's lesson' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for moral that I've seen before include "ethical principle" , "Righteous" , "Practical lesson to be drawn from an experience" , "Relating to good and evil" , "Virtuous and principled" .)