Test after master's first lesson (5)
I believe the answer is:
moral
'lesson' is the definition.
('moral' can be a synonym of 'lesson')
'test after master's first' is the wordplay.
'test' becomes 'oral' (oral is a type of examination).
'after' means one lot of letters go next to another.
'first' says to take the initial letters.
The first letter of 'masters' is 'm'.
'oral' put after 'm' is 'MORAL'.
(Other definitions for moral that I've seen before include "Relating to good and evil" , "Honourable" , "Virtuous and principled" , "Lesson of a fable" , "Righteous" .)