Rocks in water are effectively traps (4)
I believe the answer is:
reef
'rocks' is the definition.
(I've seen this before)
'water are effectively traps' is the wordplay.
'traps' says the answer is hidden in the clue (the answer is 'trapped' somewhere in the clue).
'REEF' is hidden within 'water are effectively'.
'in' is the link.
(Other definitions for reef that I've seen before include "Low-lying coral ridge" , "Coral-ridge" , "Ridge near the surface of water" , "Chain of rocks underwater" , "Coral or rock just below sea surface" .)