Sea mammal (7)
I believe the answer is:
dolphin
'sea mammal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dolphin that I've seen before include "Sea-mammal" , "Sea mammal like large porpoise, friendly towards people" , "Porpoise-like fish" , "Snall type of whale" , "Bottlenose creature?" .)