Crossword Genius

Everything is left out of poor-quality housing (3)

Ross

I believe the answer is:

sum

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'everything' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

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(Other definitions for sum that I've seen before include "Amount of money; add" , "Whole" , "Total, add" , "maths problem" , "Total; problem" .)

I've seen this clue in The Times.
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