Round piece on a necklace (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
bead
(Other definitions for bead that I've seen before include "Necklace component" , "Part of a necklace" , "Drop of moisture; holed piece of glass" , "bit off necklace, say" , "Small round piece of glass" .)