Many a test is ethical (5)
I believe the answer is:
moral
'ethical' is the definition.
('moral' can be a synonym of 'ethical')
'many a test' is the wordplay.
'many' becomes 'm' (**).
'a test' becomes 'oral' (oral is a kind of test).
'm'+'oral'='MORAL'
'is' is the link.
(Other definitions for moral that I've seen before include "Pertaining to right and wrong conduct" , "Lesson offered by story" , "ethical principle" , "Righteous" , "Lesson of a fable" .)