Clue: Plumed hat not quite right in formal dance (8)
Answer: BALMORAL
Explanation: Definition: Plumed hat
An envelope ('in') of MORA[l] ('right') minus the last letter ('not quite') in BALL ('formal dance'). A Balmoral is a bonnet with a pompon, but I find it difficult to justify the description 'plumed' (although some historically related headgear sprouted plumes).