On a chair, an evil spirit (5)
I believe the answer is:
satan
'an evil spirit' is the definition.
(thesaurus)
'on a chair' is the wordplay.
'on' is a charade indicator (letters next to each other).
'a' becomes 'an'.
'chair' becomes 'sat' (I can't explain this - if you can you should give a lot more credence to this answer).
'an' put after 'sat' is 'SATAN'.
Can you help me to learn more?
(Other definitions for satan that I've seen before include "Old Nick, Lucifer or Beelzebub" , "Fallen archangel" , "Apollyon" , "The devil, Beelzebub" , "Prince of Darkness" .)