Aquatic mammal seabird caught (3)
I believe the answer is:
orc
'aquatic mammal' is the definition.
(orc can mean a whale)
'seabird caught' is the wordplay.
'seabird' becomes 'auk' (auk is a kind of seabird).
'caught' indicates a 'sounds like' (homophone) clue (to catch something can mean to hear it).
'auk' sounds like 'ORC'.
(Other definitions for orc that I've seen before include "Imaginary creature" , "Killer-whale" , "Sea monster; or Tolkien character" , "Tolkien creature" , "Tolkien goblin" .)