Performing murder? Good! (5)
I believe the answer is:
'performing' is the definition.
('do' can be a synonym of 'perform')
'murder? good' is the wordplay.
'murder?' becomes 'do in' ('do in' can mean to kill).
'good' becomes 'g' (abbreviation).
(Other definitions for doing that I've seen before include "Impersonating" , "visiting as a tourist" , "Effecting" , "Accomplishing" , "Working" .)