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 "See 7", "Insignia for sale", "A mammal; a fastening", "Ratify", "Fasten securely; aquatic mammal", "Amphibious sea animal or kind of stamp", "Sale of insignia", "Close securely; mammal", "Bull or cow" and "Fasten with hot wax".)