Barrier historically busted (3)
I believe the answer is:
dam
'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" .)