Fable's lesson (5)
I believe the answer is:
moral
'fable's lesson' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for moral that I've seen before include "Upright; message of fable" , "Virtuous and principled" , "Relating to principles of right and wrong" , "Underlying significance of a story or event" , "Righteous" .)