Marine mammal, close (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 before)
'close' is the second definition.
(sealing is a kind of closing)
(Other definitions for seal that I've seen before include "Imprinted disk affixed to a document" , "Member of an elite force within the US Navy" , "Make tight" , "Authorise - plug" , "marine creature" .)