Possible Clues:

• Fictional material kept in Tory organisation
• Immigration of unknown number into 15 port, I gathered — is that feared by 15?
• *“Superman”
• Mineral avoided by Superman
• Fear of superhero being OK in pretty pants
• One's undoing, in slang
• Only substance to which Superman is vulnerable
• Pretty OK in Barking, but a threat to Kent
• Fictional substance in "Superman"
• Superman's weakness