All worried about the bizarre killing (6)
I believe the answer is:
lethal
'killing' is the definition.
(I've seen this before)
'all worried about the bizarre' is the wordplay.
'worried' indicates anagramming the letters (worry can mean to interfere with or disturb).
'about' indicates putting letters inside.
'bizarre' indicates an anagram.
'all' is an anagram of 'lal'.
'the' anagrammed gives 'eth'.
'lal' placed around 'eth' is 'LETHAL'.
(Other definitions for lethal that I've seen before include "describing a killer?" , "Liable to cause death" , "Deadly or mortal" , "Vicious" , "Murderous" .)