Killer in blue: die horribly! (7)
I believe the answer is:
cyanide
'killer' is the definition.
(cyanide is a lethal poison)
'blue die horribly' is the wordplay.
'blue' becomes 'cyan' (I have seen 'Greenish blue ' mean 'cyan' so perhaps 'blue' could also mean 'cyan').
'horribly' indicates an anagram (a horribly wrong version of the letters).
'die' is an anagram of 'ide'.
'cyan'+'ide'='CYANIDE'
'in' acts as a link.
(Other definitions for cyanide that I've seen before include "killer" , "Highly toxic substance" , "Nice day (anag) - poison" , "Poisonous compound" , "Extremely poisonous salt" .)