Type of knot (4)

I think it could be either:

Which length description are you looking for?:

reef

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

This is all the clue.

(Other definitions for reef that I've seen before include "Ridge of rock or sand just below sea surface" , "Free (anag.)" , "portion of sail" , "Underwater ridge or kind of knot" , "Coral structure" .)

I've seen this clue in The Telegraph and The Mirror.

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