Attack on a clique (5)

I believe the answer is:

onset

'attack' is the definition.
(archaic term for a military attack)

'on a clique' is the wordplay.
'a clique' becomes 'set' (I've seen this before).
'on'+'set'='ONSET'

(Other definitions for onset that I've seen before include "Beginning (of disease etc)" , "Start, beginning" , "Beginning (of something often unpleasant)" , "Attack using stone" , "Advent" .)

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

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