Cockney causes injury with weapons (4)
I believe the answer is:
arms
This clue is a double definition.
'cockney causes injury' is the first definition.
('harms' said with a cockney accent, dropping the 'H')
'weapons' is the second definition.
('arm' can be a synonym of 'weapon')
'with' acts as a link.
(Other definitions for arms that I've seen before include "Limbs of war" , "Shaw wrote play '. . . . and the Man'" , "George Bernard Shaw play '. . . . and the Man'" , "bombs and guns etc" , "you'll find in the gunroom" .)