Clue: Summerhouse is roughly 83% double-glazed, polished, but not dull (6)
Answer: GAZEBO
Explanation: Definition: Summerhouse
Taking the first 10 letters (about 83%) of DOUBLE GLAZED and removing DULL leaves OBEGAZ, which is an anagram of GAZEBO. Ingenious, but as I guessed the answer immediately from the definition and having the O of Oberon, this felt rather more trouble than it was worth.