Additional outcome (3)

I believe the answer is:

sum

I'm a little stuck... Click here to teach me more about this clue!

'additional outcome' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

This is all the clue.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app