Accomplishing murder? Good (5)
I believe the answer is:
doing
'accomplishing' is the definition.
(doing is a kind of accomplishing)
'murder? good' is the wordplay.
'murder?' becomes 'do in' ('do in' can mean to kill).
'good' becomes 'g' (abbreviation).
'doin'+'g'='DOING'
(Other definitions for doing that I've seen before include "deed?" , "Performing a task" , "Accomplishing" , "Working" , "Effecting" .)