Crossword Genius

Problem with German missing top meeting (3)

Ross

I believe the answer is:

sum

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'problem' is the definition.
(arithmetic problem)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for sum that I've seen before include "Whole" , "Quantity of money" , "Total, add" , "Amount, aggregate" , "Mathematical problem" .)

I've seen this clue in The Times.
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