Case on a necklace (6)
I believe the answer is:
locket
'case on a necklace' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for locket that I've seen before include "Small ornamental case-pendant" , "Case on a neck chain" , "One round neck holding picture" , "Small ornamental case on a necklace" , "Small ornamental case hung round the neck" .)