Royal crown (6)
I believe the answer is:
diadem
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">royal crown</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for diadem that I've seen before include "Crown or similar adornment" , "Light crown" , "Royal headdress" , "Crown or headband worn as sign of sovereignty" , "Crown or headband worn as symbol of sovereignty" .)