Possible Clues:

• The amount by which a bottle falls short of being full
• Unfilled part of a wine cask
• Loss of liquid by evaporation or leakage
• In full, a generation's waste
• Topped up? Not at first — time to provide liquid shortfall
• Amount of liquid in vessel after evaporation and leakage
• Beer lost through years following university students
• Amount by which a liquid container falls short of being full
• Amount needed to make a full cask
• Not quite brimful lager?