They're put up to block or protect (8)
I believe the answer is:
barriers
'they're put up to block or protect' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for barriers that I've seen before include "Objects erected to control progress or access as by crowds" , "Obstacles designed to prevent advance or progress" , "Obstructions" , "Obstacles to movement" , "They're put up to block or protect" .)