Type of knot that poses a danger to shipping? (4)

I believe the answer is:

reef

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'type of knot' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for reef that I've seen before include "Submerged ridge near the surface of the sea" , "Submerged barrier" , "Submerged rocks" , "Rocks in shallow water - kind of knot" , "Ridge of coral near the sea surface" .)

I've seen this clue in The Telegraph.

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