Sort of fever mark on body, half fatal (7)
I believe the answer is:
scarlet
'sort of fever' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'on body half fatal' is the wordplay.
'on' means one lot of letters go next to another.
'body' becomes 'scar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'half' indicates taking half.
'fatal' becomes 'lethal' (similar in meaning).
'lethal' with half the letters taken is 'let'.
'scar' next to 'let' is 'SCARLET'.
'mark' acts as a link.
Can you help me to learn more?
(Other definitions for scarlet that I've seen before include "Fallen" , "Red colour of clarets" , "Crimson" , "Cherry-red" , "Shade of red" .)