Crown: one on a queen, I appreciate that is round (5)
I believe the answer is:
tiara
'crown' is the definition.
(tiara is a type of crown)
'one on a queen i appreciate that is round' is the wordplay.
'one on' becomes 'ia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'i appreciate that' becomes 'ta' (informal way of saying thank you).
'is round' indicates putting letters inside.
'ia'+'r'='iar'
'iar' inserted into 'ta' is 'TIARA'.
Can you help me to learn more?
(Other definitions for tiara that I've seen before include "Crescent-shaped headdrsss or Pope's diadem" , "special headgear" , "Ornamental headpiece" , "Great delight" , "Type of diadem" .)