Massacre, slaughter (6)
I believe the answer is:
pogrom
This clue is a double definition.
'massacre' is the first definition.
(I've seen this in another clue)
'slaughter' is the second definition.
(I have seen 'Mass slaughter ' mean 'pogrom' so perhaps 'slaughter' could also mean 'pogrom')
(Other definitions for pogrom that I've seen before include "Mass slaughter" , "Organised massacre of Jews" , "Organised persecution of an ethnic group" , "Racial massacre" , "An organised massacre of a people" .)