A maxim spoken by many (5)
I believe the answer is:
moral
'a maxim' is the definition.
(thesaurus)
'spoken by many' is the wordplay.
'spoken' becomes 'oral' ('oral' can be similar in meaning to 'spoken').
'by' is a charade indicator (letters next to each other).
'many' becomes 'm' (**).
'oral' put after 'm' is 'MORAL'.
(Other definitions for moral that I've seen before include "Honourable" , "ethical principle" , "ethical standard" , "Pertaining to right and wrong conduct" , "Behaving ethically" .)