Dumping grounds dry up, one of those filled with sewage (7)
I believe the answer is:
cesspit
'one of those filled with sewage' is the definition.
I can't judge whether this definition defines the answer.
'dumping grounds dry up' is the wordplay.
'dumping grounds' becomes 'tips' (I can't justify this - if you can you should believe this answer much more).
'dry' becomes 'sec' (similar in meaning).
'up' is a reversal indicator (in down clue: letters go upwards).
'tips'+'sec'='tipssec'
'tipssec' backwards is 'CESSPIT'.
Can you help me to learn more?
(Other definitions for cesspit that I've seen before include "Sewage holder" , "Place for filthy water" , "Sump" , "Sewage receptacle" , "Foul and squalid place" .)