Work at school on ballet about Queen’s supreme importance (13)
I believe the answer is:
preponderance
'supreme importance' is the definition.
'work at school on ballet about queen's' is the wordplay.
'work at school' becomes 'prep' (boarding school term for homework).
'ballet' becomes 'dance'.
'about' is an insertion indicator.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'dance' enclosing 'er' is 'derance'.
'prep'+'on'+'derance'='PREPONDERANCE'
(Other definitions for preponderance that I've seen before include "In majority of" , "The lion's share" , "Greater quantity" , "Superiority in numbers" , "Predominance" .)