An original killer (4)
I believe the answer is:
cain
'killer' is the definition.
(Cain who killed Abel)
'an original' is the wordplay.
I cannot really see how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is present in the answer.
The remaining letters 'can' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cain that I've seen before include "Abel's brother" , "Eve's firstborn" , "First murderer reported in the Bible" , "The first known murderer" , "First person born in Bible" .)