Ornament Celts originally placed under rocky hill (4)
I believe the answer is:
torc
'ornament' is the definition.
(I've seen this before)
'celts originally placed under rocky hill' is the wordplay.
'originally' says to take the initial letters.
'placed under' means one lot of letters go next to another.
'rocky hill' becomes 'tor' (I've seen this before).
The initial letter of 'celts' is 'c'.
'c' put after 'tor' is 'TORC'.
(Other definitions for torc that I've seen before include "Metal neck ornament of ancient Britons" , "Celtic metal neck-ring" , "ancient necklace" , "Choker" , "One gets into a twist" .)