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

I believe the answer is:

cain

I'm a little stuck... Click here to teach me more about this clue!

'a murderer' is the definition.
(Cain who killed Abel)

'able to get out of line' is the wordplay.
'able' becomes 'can'.
'to get' means one lot of letters goes inside another.
'out of line' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'can' enclosing 'i' is 'CAIN'.

'is' is the 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.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.