Possible Clues:

• Architectural underside
• Under-surface of an arch
• Ceiling�s yielding if set up inside
• Underside of arch, stair, etc
• Undersurface (of e.g. arch)
• Row sailor back to arrive at underside of arch
• Underside of structure pliable, provided held back
• Underside of an eave
• When not at work sit around, below stairs perhaps
• 'The underside of part of a building, arch say (6)'