Stamp showing marine animal (4)
I believe the answer is:
seal
I believe this is a double definition.
'stamp' is the first definition.
('seal' can be a synonym of 'stamp')
'marine animal' is the second definition.
(I've seen this before)
'showing' is the link.
(Other definitions for seal that I've seen before include "Signet" , "Settle or decide - say fate" , "Bull or cow" , "Make tight" , "Fasten securely; aquatic mammal" .)