Deadly human being! (6)
I believe the answer is:
mortal
This clue is a double definition.
'deadly' is the first definition.
(synonyms)
'human being' is the second definition.
(as opposed to a god or immortal)
(Other definitions for mortal that I've seen before include "Destined to die" , "Very drunk < person" , "Causing death" , "Human being, subject to death" , "Sound a wound is deadly" .)