Possible Clues:

• Marine creature with a group of conservationists? It fills gap
• Boy camps in agreed pitch
• It should stop worker going on main line
• A plug for the trainee - in the main a hard worker
• Protective covering for marine mammal worker
• Material used for an airtight or watertight join
• Material for making airtight or watertight
• Eg, Fermat or Napier
• Deck covering to keep out moisture
• It may fill most of road in constituency