Eg, whale or dolphin (8)
I believe the answer is:
cetacean
This clue is a double definition.
'eg whale' is the first definition.
(whale is a kind of cetacean)
'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" , "Member of the animal order containing whales and dolphins" , "One swimming" , "Generic name for whale, dolphin or porpoise" , "Animal order including whales and dolphins" .)