Sandbank (4)
'sandbank' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reef that I've seen before include "Rock formed of coral" , "Ridge near the surface of water" , "Rocky ridge near sea surface" , "marine hazard" , "Shorten sail" .)
'sandbank' is the definition.
(I know that sandbank can be written as shoal)
This is all the clue.
(Other definitions for shoal that I've seen before include "Fish grouped" , "School of fish" , "Collection of fish" , "A great number of fish swimming together" , "Large group of fish - sandbank" .)