Possible Clues:

• Break a leg, needing support for exercise in school?
• What may be employed by summer time — underwear to keep cool, at last
• Subject with many unknowns
• Branch of maths using letters and symbols
• A large book involving subject with many unknowns
• Oddly, all agree bursar's mathematics
• Search for the unknown?
• Form of maths
• Method of calculating using symbols
• How one might sort out the problem of one's ex