A sum in teacher's head a lot? (5)

I believe the answer is:

total

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'a sum' is the definition.
(I know that add can be written as total)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for total that I've seen before include "The whole number" , "Reckon" , "Everything added together" , "Aggregate, sum" , "gross" .)

I've seen this clue in the Evening Standard.

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