Actor claims to possess this necklace (4)
I believe the answer is:
torc
'this necklace' is the definition.
(medieval neck ornament)
'actor claims to possess' is the wordplay.
'to possess' says the answer is hidden in the clue.
'TORC' is hidden in the letters of 'actor claims'.
(Other definitions for torc that I've seen before include "Metallic neck ornament" , "ancient necklace" , "Metal neck ornament of ancient Britons" , "Metal armband worn by Ancient Britons" , "Choker" .)