Regal headwear (5)
'regal headwear' 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 crescent-shaped headdress or Pope's diadem" , "Ornamental head band" , "Decorative headband" , "Jewelled headband" , "Jewelled hair-band" .)
(Other definitions for crown that I've seen before include "Coronet" , "Highest part; coin" , "Coin; royal headgear" , "Symbol of monarchy" , "Top of the head; coin" .)