Sum? Unofficial salesman offloads third (3)

I believe the answer is:

tot

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

I don't understand the remainder of the clue.

(Other definitions for tot that I've seen before include "Arithmetic for infant" , "Get the sum for the toddler" , "Add - youngster - dram" , "Add up, aggregate" , "slug?" .)

I've seen this clue in The Independent.

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