Shopping bag overturned set of scales (4)

I believe the answer is:

abac

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'shopping' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for abac that I've seen before include "Chart for calculating" , "aid in calculation" .)

I've seen this clue in The Times and The Sunday Times.

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