Cetacean mammal (5)
I believe the answer is:
whale
'cetacean mammal' is the definition.
(whale is a kind of cetacean mammal)
This is the entire clue.
(Other definitions for whale that I've seen before include "Large marine mammal" , "Aquatic mammal" , "having such a good time?" , "Largest animal" , "It breathes through a blowhole" .)