Lesson about molar (5)
I believe the answer is:
moral
'lesson' is the definition.
(as in the moral of the story)
'about molar' is the wordplay.
'about' indicates an anagram.
'molar' with letters rearranged gives 'MORAL'.
(Other definitions for moral that I've seen before include "Concerned with principles of right and wrong" , "Relating to principles of right and wrong" , "Pertaining to right and wrong conduct" , "knowing right from wrong" , "Lesson of a fable" .)