Breach of good manners (8)
I believe the answer is:
solecism
'breach of good manners' is the definition.
(instance of incorrect behaviour)
This is all the clue.
(Other definitions for solecism that I've seen before include "Social gaffe" , "Grammatical (or social) blunder" , "Grammatical mistake" , "Absurdity" , "Incongruity" .)