Relaxed next to an evil spirit (5)
'evil spirit' is the definition.
'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.
(Other definitions for satan that I've seen before include "Old Nick, Lucifer or Beelzebub", "arch-fiend", "Fallen archangel", "Fiend", "Diabolical figure".)