Possible Clues:

• Fault with 'S' not to the fore?
• Limitation in some pronunciation initially
• Saying 'th' for 's'
• One way to make a sinner thinner
• Subject for a speech therapist
• Topic in speech therapy
• Thee 33 across in a manner of thpeaking
• With this, in a manner of speaking, one might make the girl a myth while single
• Th'peak
• Speak with defective sibilants