Indiscriminate slaughter (8)
I believe the answer is:
massacre
'indiscriminate slaughter' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for massacre that I've seen before include "Slaughter of large number of people" , "Destroy" , "Murder (of many persons)" , "Wholesale slaughter" , "Large-scale indiscriminate killing" .)