Tusked sea mammal (7)
I believe the answer is:
narwhal
'tusked sea mammal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for narwhal that I've seen before include "beast" , "Cetacean with large tusk" , "Tusked sea mammal" , "Arctic toothed whale" , "aquatic mammal" .)