Performing murder? Good! (5)
I believe the answer is:
doing
'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).
'doin'+'g'='DOING'
(Other definitions for doing that I've seen before include "Impersonating" , "visiting as a tourist" , "Effecting" , "Accomplishing" , "Working" .)