Add up characters in Iliad in reverse order (3)

I believe the answer is:

sum

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

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

Can you help me to learn more?

(Other definitions for sum that I've seen before include "maths problem" , "Mathematical problem" , "Amount of money; add" , "See 25" , "Amount of money; total" .)

I've seen this clue in The Times.

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