Coral ridge (4)
I believe the answer is:
reef
'coral ridge' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for reef that I've seen before include "Kind of knot (avoided by sailors?)" , "Ridge of rock underwater" , "The Great Barrier ..., Australian natural wonder" , "Ridge of coral near the sea surface" , "Low-lying coral ridge" .)