You can count on it being at the top of a column (6)

I believe the answer is:

abacus

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'you can count on it' is the definition.
(an abacus is a tool for doing sums)

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

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(Other definitions for abacus that I've seen before include "Counting-bead device" , "An ancient counting frame with beads" , "this adds up" , "Very early calculator" , "Old counter" .)

I've seen this clue in The Guardian and The Globe and Mail.

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