Social function (4)
'social function' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ball that I've seen before include "Sphere for sport" , "Sphere - dance" , "Party" , "Round object - dance" , "form of entertainment" .)
(Other definitions for reception that I've seen before include "Entrance for office" , "First class" , "Hotel entrance area" , "A social event" , "signal quality" .)