Moral behaviour from the start in every true heart, it's common sense (6)
I believe the answer is:
ethics
'moral behaviour' is the definition.
(behaving ethically)
'start in every true heart it's common sense' is the wordplay.
'start in' indicates taking the first letters.
The first letters of 'every true heart its common sense' is 'ETHICS'.
'from the' is the link.
(Other definitions for ethics that I've seen before include "Code of practice for professional people" , "Motivation based on ideas of right and wrong" , "Codes of behaviour for business people" , "Wild and impassioned period of shouting" , "body of principles" .)