Fence (4)
'fence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for barrier that I've seen before include "dam" , "fencing?" , "Obstacle that prevents entrance" , "Construction to contain, say, crowds" , "Obstacle to progress" .)
'fence' is the definition.
(rail fence is a kind of fence)
This is the entire clue.
(Other definitions for rail that I've seen before include "British --; picture --" , "Track for wheels" , "Mode of transport; bar" , "Protest strongly (at)" , "transport system" .)