Speculation about everyone dancing in classical style (6)
I believe the answer is:
ballet
'dancing in classical style' is the definition.
'speculation about everyone' is the wordplay.
'speculation' becomes 'bet' (bet is a kind of speculation).
'about' is an insertion indicator.
'everyone' becomes 'all' (I've seen this before).
'bet' going around 'all' is 'BALLET'.
(Other definitions for ballet that I've seen before include "Artistic dance form" , "Dance entertainment" , "kind of skirt" , "The theatre of dance" , "Eg, Swan Lake" .)