The "green glass door" is a venerable puzzle/game/icebreaker/time-wasting technique where you make people guess what rule determines what objects may pass through the green glass door. It turns out the rule is this: Only things that are spelled with double letters may pass. So BEES are allowed, but WASPS are not. The MOON can get through, but not the SUN.
The answers to the clues are:
The first letter of each answer spells DOUBLE, but that's not the answer to the puzzle. Hopefully it will inspire you to look instead at what letter is doubled in each answer. It's not quite true that TONS of things can get through the green glass door, but TONNES can.