Shoals (5)
I believe the answer is:
reefs
'shoals' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reefs that I've seen before include "Ridges of rock or sand beneath sea surface" , "coral banks" , "Groups of submerged rock" , "Submerged ridges of rock or coral near the surface of water" .)