Start assault (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)

'assault' is the second definition.
(I've seen this before)

(Other definitions for onset that I've seen before include "First signs" , "Assault; beginning" , "Start - opening moments" , "Start, beginning" , "Stone (anag)" .)

I've seen this clue in The Times.

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