Where this has a mouth, there's no such river! (3)
I believe the answer is:
ply
'where this has a mouth there's no such river' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for ply that I've seen before include "This wood is laminated" , "Practise (a trade)" , "Laminate thickness" , "drama one's missed" , "A layer to work at diligently." .)