Identify first murder victim found by lake (5)
I believe the answer is:
label
'identify' is the definition.
(I know that label is a type of differentiate)
'first murder victim found by lake' is the wordplay.
'first murder victim' becomes 'Abel' (Abel was murdered by his brother Cain in the Bible).
'found by' says to put letters next to each other.
'lake' becomes 'L' (geographical abbreviation).
'abel' after 'l' is 'LABEL'.
(Other definitions for label that I've seen before include "Means of identifying" , "Tag, tab" , "Name-tag" , "Identification tag" , "Mark" .)