Claude Monet's inner spirit (5)
I believe the answer is:
demon
'spirit' is the definition.
(demon is a kind of spirit)
'claude monet's inner' is the wordplay.
'inner' indicates a hidden word (I've seen 'in' mean this).
'DEMON' is hidden in the letters of 'claude monets'.
(Other definitions for demon that I've seen before include "Fallen angel" , "Expert" , "Evildoer" , "See 3" , "beast" .)