Beginning violent attack (5)
I believe the answer is:
onset
I believe this is a double definition.
'beginning' is the first definition.
(I know that beginning can be written as onset)
'violent attack' is the second definition.
(I've seen this before)
(Other definitions for onset that I've seen before include "Start - attack" , "Inception" , "Start - opening moments" , "Advent" , "Initial stage; attack" .)