Whale or dolphin (8)
I believe the answer is:
cetacean
I believe this is a double definition.
'whale' is the first definition.
(I've seen this before)
'dolphin' is the second definition.
(I've seen this before)
(Other definitions for cetacean that I've seen before include "Encase cat with mammals like whales and dolphins" , "Generic name for whale, dolphin or porpoise" , "Animal order including whales and dolphins" , "Member of the animal order containing whales and dolphins" , "One swimming" .)