Everything is left out of poor-quality housing (3)
I believe the answer is:
sum
'everything' is the definition.
(I've seen this in another clue)
I cannot 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 "Amount of money; add" , "Whole" , "Total, add" , "maths problem" , "Total; problem" .)