Abacus piece (4)
I believe the answer is:
'abacus piece' 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 "Drop, part of rosary" , "Small round piece of glass" , "Part of a necklace" , "Drop of liquid on a surface" , "Glass ball" .)