Report of sludge in one of three rivers (4)
I believe the answer is:
ouse
'report of sludge' is the definition.
The definition suggests a singular noun which matches the answer.
'one of three rivers' is the wordplay.
'one of' becomes 'o'.
'three rivers' becomes 'use' (I can't explain this - if you can you should believe this answer much more).
'o'+'use'='OUSE'
'in' is the link.
Can you help me to learn more?
(Other definitions for ouse that I've seen before include "bird has lost its tail" , "Name of several English rivers" , "Lewes's river" , "Great river at King's Lynn" , "River of 1 across" .)