Good manners (8)
(Other definitions for courtesy that I've seen before include "Polite behaviour" , "Civility, politeness" , "Respectful and considerate behaviour" , "Good manners" , "good breeding" .)
'good manners' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for breeding that I've seen before include "Cultivating" , "Reproduction - refinement" , "strain" , "Upbringing" , "Good manners" .)