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