Sewage initially disappearing after a flush (8)
I believe the answer is:
affluent
'flush' is the definition.
(synonyms)
'sewage initially disappearing after a' is the wordplay.
'sewage' becomes 'effluent' (I've seen this before).
'initially disappearing' means to remove the first letter.
'after' says to put letters next to each other.
'effluent' with its initial letter removed is 'ffluent'.
'ffluent' put after 'a' is 'AFFLUENT'.
(Other definitions for affluent that I've seen before include "Well-heeled" , "Well-off" , "Loaded" , "Rich; flowing freely" , "Flute fan may be wealthy" .)