Call learner first murder victim (5)
I believe the answer is:
label
'call' is the definition.
(I know that call is a more specific form of the action label)
'learner first murder victim' is the wordplay.
'learner first' becomes 'l' (1st letter of 'learner').
'murder victim' becomes 'abel' (Biblical character).
'l'+'abel'='LABEL'
(Other definitions for label that I've seen before include "Name-tag" , "Mark" , "Put a name to" , "attachment to the Christmas parcel" , "Classifying name" .)