Start an attack (5)

I believe the answer is:

onset

I believe this clue is a double definition.

'start' is the first definition.
(both can mean the beginning of something)

'an attack' is the second definition.
('onset' can be a synonym of 'attack')

(Other definitions for onset that I've seen before include "Beginning (of something often unpleasant)" , "Kick-off" , "Start, beginning" , "Outbreak" , "Beginning or early stages" .)

I've seen this clue in The Globe and Mail.

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