Deadly human (6)
I believe the answer is:
mortal
I believe this clue is a double definition.
'deadly' is the first definition.
('mortal' can be a synonym of 'deadly')
'human' is the second definition.
('mortal' can be a synonym of 'human')
(Other definitions for mortal that I've seen before include "Very drunk < person" , "liable to die" , "Certain to die" , "Subject to death, like all mankind" , "Destined to die" .)