Red mark lethal? Not half! (7)
I believe the answer is:
scarlet
'red' is the definition.
(I know that red can be written as scarlet)
'mark lethal? not half' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'not half' means to take half the letters.
'lethal' halved is 'let'.
'scar'+'let'='SCARLET'
(Other definitions for scarlet that I've seen before include "Shade of red" , "Bright red of clarets" , "Cherry-red" , "Clarets (anag.)" , "Crimson" .)