Relaxed next to an evil spirit (5)
I believe the answer is:
satan
'evil spirit' is the definition.
(thesaurus)
'relaxed next to an' is the wordplay.
'relaxed' becomes 'sat' (sitting is a type of relaxing).
'next to' says to put letters next to each other.
'sat'+'an'='SATAN'
(Other definitions for satan that I've seen before include "Old Nick, Lucifer or Beelzebub" , "arch-fiend" , "Fallen archangel" , "Fiend" , "Diabolical figure" .)