Make aluminium ball (6)
I believe the answer is:
formal
'ball' is the definition.
(formal event)
'make aluminium' is the wordplay.
'make' becomes 'form' (forming is a kind of making).
'aluminium' becomes 'al' ('Al' can be a synonym of 'aluminium').
'form'+'al'='FORMAL'
(Other definitions for formal that I've seen before include "Precise - ceremonial" , "Ceremonial or stiffly polite" , "stuffy" , "black tie" , "Conforming to accepted rules or customs" .)