Related Clues:
- Make watertight assurance
- One sticks to one's post
- Close relative of the walrus
- Circus performer shut up
- Make waterproof - stamp
- Authorise - plug
- Aquatic animal - to make an impression
- Sea beast - authenticating mark
- Aquatic carnivore
- Tread heavily (on an envelope?)
- Aquatic mammal - close securely
- Tread noisily
- Philatelist's item
- Bring foot down hard
- Close tightly - mammal