What a bell buoy warns of (5)
I believe the answer is:
shoal
'what a bell buoy warns of' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for shoal that I've seen before include "shallow water" , "A great number of fish swimming together" , "School of fish" , "Swarm of fish" , "Collection of fish" .)