Many have said it's ethical (5)
I believe the answer is:
moral
'ethical' is the definition.
('moral' can be a synonym of 'ethical')
'many have said' is the wordplay.
'many' becomes 'm' (**).
'have' is a charade indicator (letters next to each other).
'said' becomes 'oral' (I've seen this before).
'm'+'oral'='MORAL'
'it's' acts as a 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" .)