Prom queen topper (5)
I believe the answer is:
tiara
'prom queen topper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tiara that I've seen before include "Ornamental headpiece" , "Item of jewellery" , "Woman's head ornament" , "expensive headwear" , "it goes to one's head!" .)