Possible Clues:

• It's used in many long-legged girls' stockings
• On the end of what's synthetic
• Dock rope material
• It debuted in a 1930s toothbrush
• Only the North could be so synthetic
• Racket string material
• Is the North only synthetic? (5)
• Many long legs sport it
• It's only synthetic below the North
• Is the North only synthetic?