Could there be a typo or two? "Costume"would fit, if the definition was "fancy dress" rather than "a fancy dress." I think perhaps what we in the U.S. call a "costume party" would be called a "fancy dress party" in the U.K.
I can't think of any way to fill in the blanks as it is, with a as the second letter, and the meaning "a fancy dress" -- that would sugg