Human ethical argument finally accepted (6)
I believe the answer is:
mortal
'human' is the definition.
(synonyms)
'ethical argument finally accepted' is the wordplay.
'ethical' becomes 'moral' (synonyms).
'finally' indicates one should take the final letters.
'accepted' is an insertion indicator.
The final letter of 'argument' is 't'.
'moral' going around 't' is 'MORTAL'.
(Other definitions for mortal that I've seen before include "Human - transitory" , "Very drunk < person" , "Human being, subject to death" , "Sound a wound is deadly" , "Destined to die" .)