Notary's mark (4)
I believe the answer is:
seal
'notary's mark' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for seal that I've seen before include "Wax stamp" , "relative of the walrus" , "Amphibious sea animal or kind of stamp" , "Aquatic mammal - close securely" , "marine creature" .)