Barriers made to be overrun (5)
I believe the answer is:
rails
'barriers' is the definition.
(rail is a kind of barrier)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for rails that I've seen before include "Barriers of horizontal bars and supports" , "Water birds" , "Wading birds - dysfunctional person has come off them" , "Train tracks" , "Makes a protest" .)