Aquatic mammal close up (4)
I believe the answer is:
seal
I believe this clue is a double definition.
'aquatic mammal' is the first definition.
(seal is a kind of aquatic mammal)
'close up' is the second definition.
(I've seen this in another clue)
(Other definitions for seal that I've seen before include "Member of an elite force within the US Navy" , "Settle or decide - say fate" , "Bull or cow" , "Mammal seen swimming" , "See 7" .)