Sentence:  “There the cause of death was soon ascertained ; the victim of this daring outrage had been stabbed to death from ear to ear with a long, sharp <WSD>instrument</WSD>, in shape like an antique stiletto, which […] was subsequently found under the cushions of the hansom. […]” 
Meaning of the word marked with <WSD> tags:  A means or agency for achieving an effect.