Morose, authoritarian noble shows heartlessly unkind attitude (8)
I believe the answer is:
meanness
'morose authoritarian noble shows heartlessly' is the definition.
I can't tell whether this defines the answer.
'unkind attitude' is the wordplay.
'unkind' becomes 'mean' (I've seen this before).
'attitude' becomes 'ness' (I can't justify this - if you can you should give a lot more credence to this answer).
'mean'+'ness'='MEANNESS'
Can you help me to learn more?
(Other definitions for meanness that I've seen before include "Lack of generosity" , "Stingy nature" , "'Stinginess, parsimony (8)'" , "significant failure of charity" , "Miserly quality" .)