Abacus part (4)
I believe the answer is:
bead
'abacus part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bead that I've seen before include "A bit of the rosary" , "Spot of dew; one threaded on necklace" , "Small round piece of glass" , "Drop, part of rosary" , "Item on a necklace" .)