Sort of knot that could be dangerous for sailors

I believe the answer is:

reef

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'sort of knot' is the definition.

I can't explain the rest of the clue.

(Other definitions for reef that I've seen before include "Sandbank" , "Ridge of rock or sand just below sea surface" , "Coral structure" , "Main bank" , "Bank of jagged rock near the surface of the sea" .)

I've seen this clue in The Daily Mail.

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