Natural barrier (4)
I believe the answer is:
reef
'natural barrier' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reef that I've seen before include "Chain of rocks near the surface of the sea" , "Coral or rock just below sea surface" , "Coral-ridge" , "Ridge of rock or sand just below sea surface" , "Coral formation - kind of knot" .)