Princess's headpiece (7)
'princess's headpiece' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tiara that I've seen before include "Ornamental headpiece" , "It might go to one's head" , "Half-crown?" , "Jewelled, ornamental headgear" , "Woman's head ornament" .)
(Other definitions for coronet that I've seen before include "Sub-lieutenant" , "Diadem" , "Peer's headgear" , "brass embellishment" , "mark of noble rank?" .)