Formal headpiece (5)
I believe the answer is:
tiara
'formal headpiece' 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 head gear" , "Jewelled headpiece" , "Jewelled headwear" , "Small crown" , "Ornamental crescent-shaped headdress or Pope's diadem" .)