The study of morals (6)
I believe the answer is:
ethics
'the study of morals' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ethics that I've seen before include "Motivation based on ideas of right and wrong" , "Principles of right and wrong" , "Codes of behaviour for business people" , "Science of morals" , "Code of practice for professional people" .)