Possible Clues:

• Machine operator altered shift
• Altered (anag) — foot-operated pedal
• Old sewer's part altered having collapsed
• Altered ground on which you might step
• One needing sole contact for power transmission
• Put your foot on the foreign pedal
• You've got to put your foot down to make one work
• But is it manipulated solely by the seamstress?
• Altered setting of foot pedal
• Thus one is made to put one's foot down