Counter full of beads? (6)

I believe the answer is:

abacus

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'counter' is the definition.
(device for counting with)

I don't understand how the rest of the clue works.

(Other definitions for abacus that I've seen before include "Mechanical calculator" , "Simple calculating device" , "As a cub (anag)" , "that one can count on" , "Beaded counting frame" .)

I've seen this clue in The Irish Times.

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