Crossword Genius

Barrier historically busted (3)

Ross

I believe the answer is:

dam

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'barrier' is the definition.
(I know that dam is a type of barrier)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for dam that I've seen before include "A mad sort of mum" , "eg Aswan" , "Horse's mother" , "Mother of quadruped" , "Female parent of an animal, especially livestock" .)

I've seen this clue in the Evening Standard.
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