Apex predator of the ocean (4)
I believe the answer is:
orca
'apex predator of the ocean' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for orca that I've seen before include "Massive killer" , "Sea creature" , "Predatory black and white toothed whale" , "grampus" , "Cor! a killer whale" .)