Possible Clues:

• Sense of humor
• 'Sounds like it's humorous in you, but it's no joke to knock on it (5,4)'
• *Not-so-humorous humerus spot
• Sensitive elbow area, and a literal hint to the circled letters
• *Sensitive spot on the elbow
• Sounds as if it might be the humerus, but it's not amusing to knock it
• Curious type of blood a person's found in part of arm
• It's not easy to see the joke of a blow there
• Curious book given to individual that touches exposed nerve
• The humerus?