Group in the know about attack (3,4)
I believe the answer is:
set upon
'attack' is the definition.
(setting upon is a kind of attacking)
'group in the know' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'in the know' becomes 'upon' (I've seen this in another clue).
'set'+'upon'='SET UPON'
'about' is the link.
(Other definitions for set upon that I've seen before include "Attack" , "Mug" , "Determined to do (something)" , "Attacked" .)