Atoll feature (4)
I believe the answer is:
reef
'atoll feature' 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 "Ridge of rock or sand just below sea surface" , "Coral or rock just below sea surface" , "Coral formation - kind of knot" , "Submerged ridge near the surface of the sea" , "Ridge of rock just below ocean surface" .)