Hard on weapons causing injuries (5)
I believe the answer is:
harms
'injuries' is the definition.
('harm' can be a synonym of 'injury')
'hard on weapons' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'weapons' becomes 'arms' ('arm' can be a synonym of 'weapon').
'h'+'arms'='HARMS'
'causing' acts as a link.
(Other definitions for harms that I've seen before include "Hurts, damages" , "Injures, damages" , "Wounds" .)