Mark Lambert, murder victim (5)
I believe the answer is:
label
'mark' is the definition.
(synonyms)
'lambert murder victim' is the wordplay.
'lambert' becomes 'L' (synonyms).
'murder victim' becomes 'abel' (biblical figure murdered by his brother Cain).
'l'+'abel'='LABEL'
(Other definitions for label that I've seen before include "Identifying tag" , "Name-tag" , "Tag or trade name" , "Designate" , "Mark" .)