The study or the science of morals (6)
I believe the answer is:
ethics
I believe this is a double definition.
'the study' is the first definition.
(ethics is a kind of study)
'the science of morals' is the second definition.
(I've seen this before)
(Other definitions for ethics that I've seen before include "Moral code" , "Codes of practice for business" , "System of morals" , "Codes of behaviour for professional people" , "Principles of right and wrong" .)