Possible Clues:

• What builders do, say, when mine lacks this passage?
• Passage with one tenor holding a note
• Nearly horizontal entrance to a mine
• By the sound of it, put it on into mine
• A channel that's denied companion access
• A key, it is for opening
• Entrance examination shunned by university
• Miner's route
• Miner's passage
• Read about escaping with it by the passage