Deadly, fatal (6)
I believe the answer is:
mortal
This clue is a double definition.
'deadly' is the first definition.
(I know that deadly can be written as mortal)
'fatal' is the second definition.
(similar in meaning)
(Other definitions for mortal that I've seen before include "Human being, subject to death" , "Very drunk < person" , "Fatal" , "Subject to death, like all mankind" , "Certain to die" .)