Chap to kill carrying large instrument (8)
I believe the answer is:
Here is my best explanation:
'instrument' is the definition.
(mandolin is a kind of instrument)
'chap to kill carrying large' is the wordplay.
'chap' becomes 'man' (I've seen this before).
'to kill' becomes 'doin' (doing in is a kind of killing**).
'carrying' means one lot of letters goes inside another.
'large' becomes 'l'.
'doin' placed around 'l' is 'dolin'.
(Other definitions for mandolin that I've seen before include "Round-backed musical instrument", "'Captain Corelli's - - '", "Musical instrument like a small guitar", "one's deployed in the kitchen", "Small guitar-like instrument", "Stringed instrument, usually played with a plectrum", "vegetable slicer", "Stringed instrument in almond", "Musical instrument like a small lute" and "it has strings attached".)