Exam following Monsieur's lesson (5)
I believe the answer is:
moral
'lesson' is the definition.
('moral' can be a synonym of 'lesson')
'exam following monsieur's' is the wordplay.
'exam' becomes 'oral' (oral exam is a kind of exam).
'following' is a charade indicator (letters next to each other).
'monsieur' becomes 'm'.
'oral' put after 'm' is 'MORAL'.
(Other definitions for moral that I've seen before include "Lesson of a fable" , "Behaving ethically" , "ethical standard" , "Relating to principles of right and wrong" , "Underlying significance of a story or event" .)