Assault; beginning (5)
I believe the answer is:
onset
I believe this clue is a double definition.
'assault' is the first definition.
(I've seen this in another clue)
'beginning' is the second definition.
(I know that onset is a type of start)
(Other definitions for onset that I've seen before include "Attack using stone" , "Beginning (of something often unpleasant)" , "Stone (anag)" , "Initial stage; attack" , "Kick-off" .)