Obscure group involved in attack (4,3)
I believe the answer is:
blot out
'obscure' is the definition.
('blot out' can be a synonym of 'obscure')
'group involved in attack' is the wordplay.
'group' becomes 'lot' (lot is a kind of group**).
'involved in' indicates putting letters inside.
'attack' becomes 'bout' (I've seen this before).
'lot' going into 'bout' is 'BLOT OUT'.
(Other definitions for blot out that I've seen before include "Disregard (a painful thought)" , "Obscure (a view)" , "Obscure completely" .)