Ornament Celts originally placed under hill (4)
I believe the answer is:
torc
'ornament' is the definition.
(I know that torc is a type of necklace)
'celts originally placed under hill' is the wordplay.
'originally' indicates taking the first letters.
'placed under' is a charade indicator (letters next to each other).
'hill' becomes 'tor' (tor is a kind of hill).
The first letter of 'celts' is 'c'.
'c' after 'tor' is 'TORC'.
(Other definitions for torc that I've seen before include "Celtic metal neck-ring" , "Rigid circular necklace" , "Metal neck ornament of ancient Britons" , "Twisted metal neck band" , "Metallic neck ornament" .)