Squalid dwelling (4)
(Other definitions for slum that I've seen before include "Area of poor housing" , "Area of bad housing" , "Squalid and overcrowded urban area" , "Squalid, overcrowded dwelling" , "Squalid neighbourhood" .)
'squalid dwelling' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hovel that I've seen before include "dump" , "shed" , "Dirty, neglected dwelling" , "Ramshackle dwelling-place" , "Grubby tumbledown dwelling" .)