Regalia, precious stones on top of one's head (5,6)
I believe the answer is:
crown jewels
'regalia' is the definition.
(crown jewels is a kind of regalia)
'precious stones on top of one's head' is the wordplay.
I cannot really understand how this works, but
'stones' could be 'jewels' (I have seen 'Precious stones ' mean 'jewels' so perhaps 'stones' could also mean 'jewels') and 'jewels' is found within the answer.
'top' could be 'crown' (crown can mean the top of something) and 'crown' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for crown jewels that I've seen before include "held in the Tower of London" , "Royal regalia" , "Royal treasure" , "National regalia" , "Monarch's regalia" .)