Code of behaviour (6)
I believe the answer is:
ethics
'code of behaviour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ethics that I've seen before include "Codes or principles for professional people" , "Code of behaviour" , "Science of moral principles" , "Codes of behaviour for professional people" , "Professional codes of behaviour.18 Serious or heartfelt" .)