Marine animal shut tight (4)
I believe the answer is:
seal
I believe this is a double definition.
'marine animal' is the first definition.
(I've seen this in another clue)
'shut tight' is the second definition.
(I've seen this in another clue)
(Other definitions for seal that I've seen before include "making an impression" , "Marine mammal for sale" , "Member of an elite force within the US Navy" , "Make waterproof - stamp" , "Stamped wax; mammal" .)