Stamp on marine mammal? (4)
I believe the answer is:
seal
I believe this is a double definition.
'stamp' is the first definition.
(seal is a kind of stamp)
'marine mammal?' is the second definition.
(I've seen this before)
'on' acts as a link.
(Other definitions for seal that I've seen before include "Marine mammal for sale" , "making an impression" , "relative of the walrus" , "A mammal; a fastening" , "Make waterproof - stamp" .)