Mark exam in lesson (5)
I believe the answer is:
moral
'lesson' is the definition.
('moral' can be a synonym of 'lesson')
'mark exam' is the wordplay.
'mark' becomes 'm'.
'exam' becomes 'oral' (oral exam is a kind of exam).
'm'+'oral'='MORAL'
'in' is the link.
(Other definitions for moral that I've seen before include "Honourable" , "Relating to good and evil" , "Lesson of a fable" , "Relating to principles of right and wrong" , "Behaving ethically" .)