Fire breather maybe, but he'll be half dead by Monday (5)
I believe the answer is:
demon
'fire breather maybe but he'll' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'half dead by monday' is the wordplay.
'half' means to take half the letters.
'by' is a charade indicator (letters next to each other).
'monday' becomes 'Mon' (short for Monday).
'dead' halved is 'de'.
'de'+'mon'='DEMON'
'be' acts as a link.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "Evil spirit, devil" , "malignant spirit" , "Inspiring force" , "Imp" , "Genius" .)