Just a maxim (5)
I believe the answer is:
moral
This clue is a double definition.
'just' is the first definition.
(moral can mean just or fair)
'a maxim' is the second definition.
(thesaurus)
(Other definitions for moral that I've seen before include "Pertaining to right and wrong conduct" , "Underlying significance of a story or event" , "Lesson of a fable" , "Lesson that can be drawn from a story" , "Relating to principles of right and wrong" .)