Decent mark on exam (5)
I believe the answer is:
moral
'decent' is the definition.
(thesaurus)
'mark on exam' is the wordplay.
'mark' becomes 'm'.
'on' says to put letters next to each other (in a down clue, letters appear on others).
'exam' becomes 'oral' (spoken academic test).
'm'+'oral'='MORAL'
(Other definitions for moral that I've seen before include "Pertaining to right and wrong conduct" , "Honourable" , "ethical principle" , "Practical lesson to be drawn from an experience" , "Virtuous and principled" .)