Crossword Genius

No sire, it's across the river! (3)

Ross

I believe the answer is:

dam

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But I can't explain it at all! Could you help me understand?

(Other definitions for dam that I've seen before include "eg Aswan" , "A mad sort of mum" , "Hold back; mother" , "Mother - barrier" , "Female parent; obstruction" .)

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