Possible Clues:

• Confess and reveal one diamond, perhaps
• The only club, perhaps, one has not arranged to join
• Wearing vest and just one diamond?
• Individual wearing a T-shirt
• One put weight on - only one
• One object only
• Sounds enough to make one wait for one in hand
• Only card held by Defoe's officer
• Carol spilled the beans, all alone
• Lone example