Name first murder victim on line (5)
I believe the answer is:
label
'name' is the definition.
(I know that name is a more specific form of label)
'murder victim on line' is the wordplay.
'murder victim' becomes 'abel' (Biblical character).
'on' means one lot of letters go next to another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'abel' put after 'l' is 'LABEL'.
'first' acts as a link.
(Other definitions for label that I've seen before include "Name-tag" , "Identification tag" , "Means of identifying" , "Classifying name" , "Tag or designation" .)