Problem about large area of poor housing (4)
I believe the answer is:
slum
'area of poor housing' is the definition.
(I've seen this before)
'problem about large' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'about' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'sum' enclosing 'l' is 'SLUM'.
(Other definitions for slum that I've seen before include "Over-crowded run-down city area" , "Ghetto" , "Area of poor housing" , "Squalid, overcrowded dwelling" , "Squalid and overcrowded urban area" .)