Expertness of second murder (5)
I believe the answer is:
skill
'expertness' is the definition.
(thesaurus)
'second murder' is the wordplay.
'second' becomes 's' (common abbreviation).
'murder' becomes 'kill' (I've seen this before).
's'+'kill'='SKILL'
'of' acts as a link.
(Other definitions for skill that I've seen before include "Knack" , "Prowess" , "Special ability" , "Aptitude" , "Expertise, dexterity" .)