Courteous behaviour (8)
I believe the answer is:
chivalry
'courteous behaviour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chivalry that I've seen before include "Mediaeval knightly code" , "Gentlemanliness" , "Code of behaviour followed by medieval knights" , "Courtesy and kind behaviour towards weaker people" , "Knightly behaviour" .)