Waste matter goes here, contained by river banks (6)
I believe the answer is:
urinal
'waste matter goes here' is the definition.
(where urine goes)
'contained by river banks' is the wordplay.
'contained by' becomes 'in' (being in something can mean being contained by it).
'river' becomes 'ural' (Russian river).
'banks' means one lot of letters goes inside another (bank can mean to surround or border).
'in' placed into 'ural' is 'URINAL'.
(Other definitions for urinal that I've seen before include "Sanitary fitting" , "public convenience" , "waste disposal unit" .)