Crossword Genius

Started to live with a killer (5)

Ross

I believe the answer is:

begun

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'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" .)

I've seen this clue in the Evening Standard.
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