Many have spoken of what is ethical (5)
I believe the answer is:
moral
'is ethical' is the definition.
(synonyms)
'many have spoken' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'have' means one lot of letters go next to another.
'spoken' becomes 'oral' (similar in meaning).
'm'+'oral'='MORAL'
'of what' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for moral that I've seen before include "Practical lesson to be drawn from an experience" , "Honourable" , "Concerned with principles of right and wrong" , "Lesson offered by story" , "knowing right from wrong" .)