Possible Clues:

• Pronounced defect
• Problem with 16 Down
• Do a homophobic impersonation, say
• Struggle with "s" sounds when speaking
• Have a problem ordering sirloin steak?
• That's enough to make one thick with sick
• Result of a lost front tooth, perhaps
• Thpeaker's thtumbling block
• Sing a thing with this
• Daffy's impediment