Started to live with a killer (5)
I believe the answer is:
begun
'started' is the definition.
(to start is to begin)
'live with a killer' is the wordplay.
'live' becomes 'be' (synonyms).
'with' is a charade indicator (letters next to each other).
'a killer' becomes 'gun' (I can't justify this - if you can you should give a lot more credence to this answer).
'be'+'gun'='BEGUN'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for begun that I've seen before include "Opened" , "(Has) started" , "Commenced" .)