Deathly (6)
I believe the answer is:
deadly
'deathly' is the definition.
(I know that deadly can be written as deadly)
This is all the clue.
(Other definitions for deadly that I've seen before include "sniper aspires to be" , "Lethal or mortal" , "Lethal, could be fatal" , "Lethal, capable of causing fatality" , "Lethal, capable of causing fatal injury" .)