River associated with a waste product (4)
I believe the answer is:
urea
'waste product' is the definition.
(I've seen this before)
'river associated with a' is the wordplay.
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'associated with' means one lot of letters go next to another.
'ure'+'a'='UREA'
(Other definitions for urea that I've seen before include "Waste matter" , "Waste material" , "Human waste product" , "Compound found in urine, synthesised as a fertiliser" , "organic compound" .)