Small river dam (4)
I believe the answer is:
weir
'small river dam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for weir that I've seen before include "Feature of river" , "Dam on a stream to control water level or flow" , "Low dam built across a stream" , "type of block" , "fish may be trapped in this enclosure" .)