Mass slaughter Macbeth worked to cover over (8)
I believe the answer is:
hecatomb
'mass slaughter' is the definition.
(I've seen this before)
'macbeth worked to cover over' is the wordplay.
'worked' is an anagram indicator.
'to cover' is an insertion indicator.
'over' becomes 'o' (cricket abbreviation).
'macbeth' anagrammed gives 'hecatmb'.
'hecatmb' going around 'o' is 'HECATOMB'.
(Other definitions for hecatomb that I've seen before include "major sacrifice" , "sacrifice victims" , "Large-scale sacrifice" , "great public sacrifice" , "big sacrifice" .)