The devil's half dead for a day! (5)
I believe the answer is:
demon
'the devil's' is the definition.
(synonyms)
'half dead for a day' is the wordplay.
'half' indicates taking half.
'for' is a charade indicator (letters next to each other).
'a day' becomes 'mon' (abbreviation for Monday).
'dead' with half the letters taken is 'de'.
'de'+'mon'='DEMON'
(Other definitions for demon that I've seen before include "See 3" , "Expert" , "malignant spirit" , "Genius" , "Skilful person" .)