Trademark left Cain’s victim (5)
I believe the answer is:
label
'trademark' is the definition.
(I know that trademark is a more specific form of the action label)
'left cain's victim' is the wordplay.
'left' becomes 'l' (common abbreviation).
'cain's victim' becomes 'abel' (I've seen this before).
'l'+'abel'='LABEL'
(Other definitions for label that I've seen before include "Tag or trade name" , "Identify" , "Identifying tag" , "Means of identifying" , "attachment to the Christmas parcel" .)