Notary's device (4)
I believe the answer is:
seal
'notary's device' 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 "Sea-dweller can produce a wax impression" , "Sea mammal in sale" , "A mammal; a fastening" , "Imprinted disk affixed to a document" , "Aquatic mammal - close securely" .)