Massacre earl’s thug arranged (9)
I believe the answer is:
slaughter
'massacre' is the definition.
(I know that massacre can be written as slaughter)
'earl's thug arranged' is the wordplay.
'arranged' indicates an anagram.
'earls'+'thug'='earlsthug'
'earlsthug' anagrammed gives 'SLAUGHTER'.
(Other definitions for slaughter that I've seen before include "Kill (animals) for food" , "Butchery" , "Widespread killing" , "Wholesale killing" , "Mass killing or carnage" .)