Crossword Genius

Unusual, having day off to see dam (4)

Ross

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" .)

I've seen this clue in The Times.
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