Mortal (6)
(Other definitions for lethal that I've seen before include "Killing" , "bringing about the end?" , "describing a killer?" , "--- Weapon, Mel Gibson film" , "very nasty indeed" .)
'mortal' is the definition.
(I know that mortal can be written as deadly)
This is the entire clue.
(Other definitions for deadly that I've seen before include "Lethal or mortal" , "sniper aspires to be" , "Belladonna" , "Lethal, capable of killing" , "Lethal, capable of causing fatal injury" .)
'mortal' is the definition.
(synonyms)
This is all the clue.
(Other definitions for human that I've seen before include "Not divine, animal or mechanical" , "Of the species of homo sapiens" , "Man, woman or child" , "Like people" , "Anthropoid" .)
(Other definitions for fatal that I've seen before include "Unavoidable" , "Bringing ruin" , "making one late" , "critical" , "Causing death, say accident" .)
(Other definitions for mere that I've seen before include "Nothing more than a small pond" , "Former lake" , "President Hollande's mother?" , "Unimpressive" , "Lake - French mother" .)