Animal is close (4)
I believe the answer is:
seal
This clue is a double definition.
'animal' is the first definition.
(I have seen 'Marine animal' mean 'seal' so perhaps 'animal' could also mean 'seal')
'close' is the second definition.
(sealing is a kind of closing)
'is' is the link.
(Other definitions for seal that I've seen before include "relative of the walrus" , "Amphibious sea animal or kind of stamp" , "Secure against leakage" , "Make waterproof - stamp" , "Close up - marine mammal" .)