Chap to kill carrying large instrument (8)
I believe the answer is:
mandolin
'instrument' is the definition.
(mandolin is a kind of instrument)
'chap to kill carrying large' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'to kill' becomes 'doin' (to do in is to kill or assassinate).
'carrying' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'doin' enclosing 'l' is 'dolin'.
'man'+'dolin'='MANDOLIN'
(Other definitions for mandolin that I've seen before include "Small guitar-like instrument" , "Musical instrument like a small guitar" , "one's deployed in the kitchen" , "It takes some pluck to use this" , "Stringed instrument, usually played with a plectrum" .)