One retaliating with extremes of violence in rage (7)
I believe the answer is:
avenger
'one retaliating' is the definition.
(I've seen this before)
'extremes of violence in rage' is the wordplay.
'extremes of' suggests removing the centre.
'in' indicates putting letters inside.
'rage' becomes 'anger' (I've seen this before).
'violence' with its centre taken out is 've'.
've' put within 'anger' is 'AVENGER'.
'with' is the link.
(Other definitions for avenger that I've seen before include "One seeking retribution" , "Bringer of retribution" , "Someone seeking to settle a score" , "Retaliator" , "One settles score" .)