Everyone in fancy dance (6)
I believe the answer is:
ballet
'dance' is the definition.
(ballet is a kind of dance)
'everyone in fancy' is the wordplay.
'everyone' becomes 'all' (all people).
'in' is an insertion indicator.
'fancy' becomes 'bet' (I am not sure about this - if you are sure you should believe this answer much more).
'all' inserted into 'bet' is 'BALLET'.
Can you help me to learn more?
(Other definitions for ballet that I've seen before include "old madrigal" , "kind of skirt" , "Dance entertainment" , "Theatrical dance" , "The Nutcracker or Giselle, say" .)