Tot in poor area beginning to lose out (3)
I believe the answer is:
sum
'tot' is the definition.
(I know that tot can be written as sum)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sum that I've seen before include "Quantity obtained by addition" , "Amount, aggregate" , "pupils find it a problem" , "Arithmetical problem" , "All one has" .)