Crossword Genius

In total, a problem (3)

Ross

I believe the answer is:

sum

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

This could be a double definition.

'in total' is the first definition.
'sum' can be an answer for 'total' (I've seen this before). I'm not sure about the 'in' bit.

'a problem' is the second definition.
(arithmetic problem)

Can you help me to learn more?

(Other definitions for sum that I've seen before include "All one has" , "Aggregate, total" , "Total, add" , "See 25" , "Amount of money; total" .)

I've seen this clue in the Evening Standard.
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