Attack supported by group (5)

I believe the answer is:

onset

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

'supported by group' is the wordplay.
'supported by' becomes 'on'.
'group' becomes 'set' (set is a kind of group).
'on'+'set'='ONSET'

(Other definitions for onset that I've seen before include "Beginning; military attack" , "First signs" , "Initial stage; attack" , "Storming" , "origin" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app