Clue: HRH's 'carbuncle'? His mum entering bore up (5)
Answer: TERAS
Explanation: Definition: carbuncle
**ER** ("HRH's mum") *entering* **<=SAT** ("bore" *up*) 
*A teras is a monstrosity or carbuncle, which is a description HRH Prince Charles has used to describe some modern architecture.