Trick-or-treaters in white (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
ghosts
(Other definitions for ghosts that I've seen before include "Spirits or apparitions" , "Apparitions of a dead people" , "Writes for another; Ibsen play" , "Spectres, wraiths" , "Spooks, spectres" .)