Such discourtesy is trivial (6)
I believe the answer is:
slight
This could be a double definition.
'such discourtesy' is the first definition.
'slight' can be an answer for 'discourtesy' (slight is a kind of discourtesy). I am unsure of the 'such' bit.
'trivial' is the second definition.
(I've seen this before)
'is' acts as a link.
Can you help me to learn more?
(Other definitions for slight that I've seen before include "Inconsiderable - insult" , "Insult, affront" , "Affront of delicate sort" , "Barely perceptible" , "Aspersion" .)