Clue: Equivalent of dung in US river? First being removed disguises giggle (10)
Answer: OVERSLAUGH
Explanation: Definition: Equivalent of dung in US river?
Dung > dune 
[*first being removed*] (c)**OVERS** ("disguises") + **LAUGH **("giggle")
 
*An overslaugh is a bar in a river in North America.