Possible Clues:

• Horned viper I reckon to start with H
• Hook, initially, and how he lived his life
• Higgins taught Eliza to like Captain Kidd
• Pronounce with full breathing
• Higgins's initial challenge to Eliza
• In a criminal way, to a pronounced extent
• Having lost my head, I'm a growing hazard
• Pronounce an H
• That's enough to make the serpent cross to a pronounced extent
• Public enemy to a pronounced extent