Well-known demon or spirit (8)
I believe the answer is:
familiar
I believe this clue is a triple definition!
'well-known' is the first definition.
(similar in meaning)
'demon' is the second definition.
(thesaurus)
'or' acts as a link.
'spirit' is the third definition.
(I know that familiar is a type of spirit)
(Other definitions for familiar that I've seen before include "spirit" , "Witch's companion" , "Au fait" , "Well-known; usual" , "routine" .)