Killing gone mad, faction said (8)
I believe the answer is:
genocide
'killing' is the definition.
(I know that genocide is a type of killing)
'gone mad faction said' is the wordplay.
I cannot really understand how this works, but
an anagram of 'gone' is 'geno' which is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for genocide that I've seen before include "Racial murder" , "Extermination of ethnic group or nationality" , "Racial extermination" , "Ethnic cleansing" , "Deliberate extermination of a race of people" .)