Atoll (4)
I believe the answer is:
reef
'atoll' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for reef that I've seen before include "Coral formation - kind of knot" , "The sort of knot" , "Type of knot" , "Submerged ridge of rock or coral near the surface" , "Coral-ridge" .)