A knot seamen consider dangerous (4)

I believe the answer is:

reef

I'm a little stuck... Click here to teach me more about this clue!

'a knot' is the definition.
(reef knot is a kind of knot)

I cannot understand how the rest of the clue works.

(Other definitions for reef that I've seen before include "Submerged barrier" , "Type of knot" , "Rock in shallow water" , "Kind of knot (avoided by sailors?)" , "Rocks in shallow water - kind of knot" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app