No afternoon exam could be unethical (6)
I believe the answer is:
amoral
'be unethical' is the definition.
(I've seen this before)
'no afternoon exam' is the wordplay.
'no afternoon' becomes 'am' (I can't justify this - if you can you should believe this answer much more).
'exam' becomes 'oral' (spoken academic test).
'am'+'oral'='AMORAL'
'could' is the link.
Can you help me to learn more?
(Other definitions for amoral that I've seen before include "Ignoring what's right and wrong" , "Without sense of right and wrong" , "Without ethics" , "Blind to right and wrong" , "Ignoring ethical considerations" .)