A social event including cake, lots of it (9)
I believe the answer is:
abundance
'lots of it' is the definition.
(abundant amounts of something)
'a social event including cake' is the wordplay.
'social event' becomes 'dance' (a dance is a type of social event).
'including' means one lot of letters goes inside another.
'cake' becomes 'bun' (bun is a type of cake).
'a'+'dance'='adance'
'adance' enclosing 'bun' is 'ABUNDANCE'.
(Other definitions for abundance that I've seen before include "great deal" , "Nine tricks" , "Ban due can be plenty" , "Plethora" , "call at whist" .)