Kind of knot that sailors try to avoid (4)
I believe the answer is:
reef
'kind of knot' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for reef that I've seen before include "Free (anag.)" , "Chain of rocks underwater" , "Chain of rocks near the surface of the sea" , "Ridge of rock or sand just below sea surface" , "Main bank" .)