Displaying curiosity about royal ornament (4,4)
I believe the answer is:
nose ring
'ornament' is the definition.
'displaying curiosity about royal' is the wordplay.
'displaying curiosity' becomes 'nosing' (being nosy).
'about' indicates putting letters inside.
'royal' becomes 'er' (abbreviation for Elizabeth Regina).
'nosing' going around 'er' is 'NOSE-RING'.
(Other definitions for nose ring that I've seen before include "jewellery" , "Leading band" .)