Marine ecosystem (4)
I believe the answer is:
reef
'marine ecosystem' 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 near the surface of water" , "Rock in shallow water" , "Ridge of coral near the sea surface" , "Coral formation - kind of knot" , "Kind of knot (avoided by sailors?)" .)