Atone by breaking knife (7)
I believe the answer is:
bayonet
'knife' is the definition.
(bayonet is a kind of knife)
'atone by breaking' is the wordplay.
'breaking' indicates an anagram.
'atone'+'by'='atoneby'
'atoneby' is an anagram of 'BAYONET'.
(Other definitions for bayonet that I've seen before include "Stabbing weapon" , "Pointed weapon" , "Blade fixed to a rifle" , "Dagger attached to 26 Across" , "Dagger fixed to rifle" .)