Related Clues:

• Equipment put round most of the perimeter
• Hard maths topic written up to find circumference
• Hard bit of maths — set about finding measurement
• Saddle strap, right when adjusted
• Item tightened by jockey needing high tension apparatus set up
• Wobbly right, measure round waist
• Costumer's measurement
• Right to run around middle distance
• Circumference right after adjustment
• Circumference measurement
• Hard branch of maths? On reflection, it's the circumference