... heartlessviolent reaction (8)
I believe the answer is:
backlash
'reaction' is the definition.
(I know that backlash is a type of reaction)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for backlash that I've seen before include "Strong negative reaction to a development" , "Violent reaction to event, like reverse whip" , "Repercussion" , "Provoked adverse reaction" , "negative comments?" .)