Start violent attack (5)
I believe the answer is:
onset
I believe this clue is a double definition.
'start' is the first definition.
(onset is a kind of start)
'violent attack' is the second definition.
(I've seen this before)
(Other definitions for onset that I've seen before include "Storming" , "First signs" , "starting?" , "Appearance of" , "Assault; start" .)