Crossword Genius

Tot in poor area beginning to lose out (3)

Ross

I believe the answer is:

sum

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'tot' is the definition.
(I know that tot can be written as sum)

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

Can you help me to learn more?

(Other definitions for sum that I've seen before include "Quantity obtained by addition" , "Amount, aggregate" , "pupils find it a problem" , "Arithmetical problem" , "All one has" .)

I've seen this clue in The Telegraph.
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