Knot sailors try to avoid (4)
I believe the answer is:
reef
'knot' is the definition.
(reef knot is a kind of knot)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for reef that I've seen before include "The sort of knot" , "Chain of rocks underwater" , "Submerged barrier" , "The Great Barrier ..., Australian natural wonder" , "Ridge of rock or sand just below sea surface" .)