Formal dance; sphere (4)
I believe the answer is:
ball
This clue is a double definition.
'formal dance' is the first definition.
(I've seen this before)
'sphere' is the second definition.
(I know that ball is a type of sphere)
(Other definitions for ball that I've seen before include "Spherical object" , "Party" , "Old song: 'After the .... was over'" , "form of entertainment" , "All-round" .)