Attack is concerning group (5)
I believe the answer is:
onset
'attack' is the definition.
(I know that attack can be written as onset)
'concerning group' is the wordplay.
'concerning' becomes 'on' ().
'group' becomes 'set' (both can mean a collection).
'on'+'set'='ONSET'
'is' is the link.
(Other definitions for onset that I've seen before include "Start, beginning" , "Inception" , "Start - attack" , "Start - notes (anag)" , "Stone (anag)" .)