Deadly poison (7)
There are a few possibilities for this:
prussic acid
'deadly poison' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cyanide that I've seen before include "Highly toxic substance" , "Nice day (anag) - poison" , "Poisonous compound" , "killer" , "Toxic salt" .)
(Other definitions for arsenic that I've seen before include "killer" , "As briefly identified" , "Brittle grey-green semi-metal, atomic number 33" , "Toxic metalloid element" , ", it goes with old lace" .)