Marine mammal shut up (4)
I believe the answer is:
seal
I believe this clue is a double definition.
'marine mammal' is the first definition.
(I've seen this in another clue)
'shut up' is the second definition.
(I've seen this in another clue)
(Other definitions for seal that I've seen before include "Fasten with hot wax" , "Sea beast - authenticating mark" , "Stamped wax; mammal" , "Mammal; impressed wax" , "Fasten securely; aquatic mammal" .)