Weapon for one involved in attack shortly (7)
I believe the answer is:
assegai
'weapon' is the definition.
(assegai is a kind of weapon)
'for one involved in attack shortly' is the wordplay.
'for one' becomes 'eg' (I've seen this in other clues).
'involved in' is an insertion indicator.
'attack' becomes 'assail' ('assail' can be a synonym of 'attack').
'shortly' means to remove the last letter.
'assail' with its final letter taken away is 'assai'.
'eg' inserted inside 'assai' is 'ASSEGAI'.
(Other definitions for assegai that I've seen before include "Wooden spear of southern Africa" , "African weapon" , "A Zulu war spear" , "something pointed" , "Zulu fighting spear" .)