Rage can break out, leading to mass slaughter (7)
I believe the answer is:
carnage
'mass slaughter' is the definition.
(I've seen this before)
'rage can break out' is the wordplay.
'break out' indicates an anagram.
'rage'+'can'='ragecan'
'ragecan' anagrammed gives 'CARNAGE'.
'leading to' is the link.
(Other definitions for carnage that I've seen before include "Widespread killing" , "Bloody slaughter and butchery" , "Mass slaughter" , "Bloody slaughter as in battle" , "Killing of large numbers" .)