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Sort of knot ship captains may not appreciate (4)

Ross

I believe the answer is:

reef

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'sort of knot' is the definition.

I can't explain the remainder of the clue.

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(Other definitions for reef that I've seen before include "offshore bank" , "The sort of knot" , "Free (anag.)" , "Atoll protector" , "Ridge of rock underwater" .)

I've seen this clue in The Hindu.
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