Main charity served up something sparkling (6)
I believe the answer is:
Here is my best explanation:
'something sparkling' is the definition.
'main charity served up' is the subsidiary indication.
'main' becomes 'med' ('main' can mean the sea - eg Mediterranean or Med).
'charity' becomes 'aid'.
'served up' shows that the letters should be reversed in order.
'medaid' back-to-front is 'diadem'.
(Other definitions for diadem that I've seen before include "Kingly crown", "Ornamental jewelled headdress", "Royal headdress", "Tiara", "Crown or headband worn as symbol of sovereignty", "Light crown", "Small jewelled crown", "Crown or similar adornment", "Crown or headband for sovereign" and "A crowning glory".)