First of sons he once treasured is killed (4)
I believe the answer is:
shot
'killed' is the definition.
(shooting is a kind of killing)
'first of sons he once treasured' is the wordplay.
'first of' says to take the initial letters.
The first letters of 'sons he once treasured' is 'SHOT'.
'is' acts as a link.
(Other definitions for shot that I've seen before include "Ruined; photograph" , "Fired; (be) rid (of)" , "Ruined; photo" , "Charge" , "Sound of gun" .)