Cruelty making one unhappy is last thing kingdom needs (6)
I believe the answer is:
sadism
'cruelty' is the definition.
(deriving pleasure from cruelty)
'unhappy is last thing kingdom needs' is the wordplay.
I cannot really see how this works, but
'unhappy' could be 'sad' (associated in meaning) and 'sad' is present in the answer.
'is' is present in the answer.
A single letter 'm' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'making one' is the link.
Can you help me to learn more?
(Other definitions for sadism that I've seen before include "Enjoyment taken in inflicting pain or cruelty" , "The love of cruelty" , "Sexual pleasure from hurting others" , "Dim ass (anag)" , "Love of inflicting pain" .)