Cetacean that kills seabird, by the sound of it (3)
I believe the answer is:
orc
'cetacean that kills' is the definition.
(orc can mean an orca, a killer whale)
'seabird by the sound of it' is the wordplay.
'seabird' becomes 'auk' (auk is a kind of seabird).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'auk' is a homophone of 'ORC'.
(Other definitions for orc that I've seen before include "Mythical monster" , "warlike figure" , "Tolkienian monster" , "Imaginary creature" , "Ugly creature" .)