A murderer is able to get out of line (4)

I believe the answer is:

cain

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'a murderer' is the definition.
(Biblical figure)

'able to get out of line' is the wordplay.
'able' becomes 'can'.
'to get' is an insertion indicator.
'out of line' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'can' enclosing 'i' is 'CAIN'.

'is' acts as a link.

Can you help me to learn more?

(Other definitions for cain that I've seen before include "Brother of Abel" , "Adam's son" , "First person born in Bible" , "The first known murderer" , "Eve's firstborn" .)

I've seen this clue in the Evening Standard.

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