Barriers to keep in or out (6)
I believe the answer is:
fences
'barriers to keep in or out' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fences that I've seen before include "Barriers to keep in or out" , "Uses a foil" , "Dealers in stolen property" , "'Good ...... made good neighbours'" , "Engages in swordplay or processes stolen goods" .)