Many have said it's ethical (5)
I believe the answer is:
moral
'ethical' is the definition.
(synonyms)
'many have said' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'have' means one lot of letters go next to another.
'said' becomes 'oral' (both can mean spoken).
'm'+'oral'='MORAL'
'it's' is the link.
(Other definitions for moral that I've seen before include "Behaving ethically" , "Underlying significance of a story or event" , "Pertaining to right and wrong conduct" , "Concerned with principles of right and wrong" , "Relating to good and evil" .)