Possible Clues:

• The nth one in the works is a home for a 21-down
• Colonial mound
• High-rise accommodation for soldiers and others
• Pile of earth left by workers?
• Social insect nest
• Mound built by a colony
• Run away from an exhilarating experience in colony
• Mound formed by nesting insects
• Everyone swallows obscure hint the queen's home
• A problem besetting one of many a colony of insects