A selfish characteristic, no argument about that (6)
I believe the answer is:
agreed
'a selfish characteristic no' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'argument about that' is the wordplay.
I cannot really understand how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found within the answer.
The remaining letters 'aged' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "Corresponded" , "Mutually settled" , "Harmonised" , "Of one mind" , "OK, fine!" .)