Abacus slider (4)
I believe the answer is:
bead
'abacus slider' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bead that I've seen before include "Necklace element" , "Necklace piece; droplet" , "Round piece of glass, etc" , "Item on a necklace" , "Small round piece of glass" .)