Largest member of the dolphin family (4)
I believe the answer is:
orca
'largest member of the dolphin family' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for orca that I've seen before include "Killer whales" , "swimmer in black and white" , "Cor! a killer whale" , "grampus" , "Killer-whale" .)