Stamp showing marine animal (4)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'stamp' is the first definition.
(seal is a kind 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 "Fasten with hot wax", "Settle or decide - say fate", "Make airtight", "A mammal; one that secures", "Secure against leakage", "Close - (performing?) animal", "Marine animal or sign of authenticity", "Close tightly - mammal", "Secure; aquatic animal" and "Wax stamp".)