Low dam across a stream (4)
I believe the answer is:
weir
'low dam across a stream' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for weir that I've seen before include "Small dam to regulate water flow" , "Low dam built across a stream" , "Madagascan primate" , "fish may be trapped in this enclosure" , "Something that may be seen across river" .)