Tot crosses line in poor area (4)
I believe the answer is:
slum
'poor area' is the definition.
'tot crosses line' is the wordplay.
'tot' becomes 'sum' ('sum' can be a synonym of 'tot').
'crosses' is an insertion indicator.
'line' becomes 'l'.
'sum' placed around 'l' is 'SLUM'.
'in' acts as a link.
(Other definitions for slum that I've seen before include "Run-down area of city with tenements" , "Squalid and overcrowded urban area" , "Area of poor housing" , "Squalid neighbourhood" , "Squalid, overcrowded dwelling" .)