Leader of men said to be ethical (5)
I believe the answer is:
'ethical' is the definition.
('moral' can be a synonym of 'ethical')
'leader of men said' is the wordplay.
'leader of' indicates taking the first letters.
'said' becomes 'oral' (I've seen this before).
The initial letter of 'men' is 'm'.
'to be' is the link.
(Other definitions for moral that I've seen before include "Practical lesson to be drawn from an experience" , "Relating to good and evil" , "Concerned with principles of right and wrong" , "knowing right from wrong" , "Upright; message of fable" .)