Sudden and severe onset of trouble (9)
I believe the answer is:
onslaught
'sudden and severe onset of trouble' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for onslaught that I've seen before include "Hunt goals (anag) - offensive" , "Sally" , "Aggressive action" , "Sudden and severe onset of trouble" , "Vicious attack" .)