Mortal (5)
(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" .)
(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" .)