Second murdered expert (7)
I believe the answer is:
skilled
'expert' is the definition.
('skilled' can be similar in meaning to 'expert')
'second murdered' is the wordplay.
'second' becomes 's' (common abbreviation).
'murdered' becomes 'killed' (I've seen this before).
's'+'killed'='SKILLED'
(Other definitions for skilled that I've seen before include "Adept, clever" , "Competent - experienced" , "Highly trained" , "Showing great aptitude" , "Able" .)