Submerged ridge of rock or coral (4)
I believe the answer is:
reef
'submerged ridge of rock or coral' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reef that I've seen before include "Rock in shallow water" , "Type of knot" , "Submerged ridge of rock or coral near the surface" , "The sort of knot" , "sailors consider dangerous" .)