Unusual, having day off to see dam (4)
I believe the answer is:
weir
'dam' is the definition.
(a weir is a type of dam)
I don't understand how the rest of the clue works.
(Other definitions for weir that I've seen before include "Small dam built across river to regulate flow" , "Low dam built across a stream" , "Madagascan primate" , "Dam on a stream to control water level or flow" , "Feature of river" .)