Possible Clues:

• How fifty ones get in a knot onfoot
• Ones get done up by the foot
• Scale for boot fasteners
• Scale for shoe securers
• How fifty ones may get knotted
• They could be done up if they were under the piano
• Fifty singles get done up
• Adds punch to punch?
• Fifty-one's tied-up
• Cords for fastening shoes