Grandpa entertaining party over sponsor? (9)
I believe the answer is:
godfather
'sponsor?' is the definition.
(I know that Godfather is a type of godparent)
I don't understand the rest of the clue.
(Other definitions for godfather that I've seen before include "Mafia chief or spiritual parent" , "He sponsors a child at baptism" , "He's a sponsor at Baptism" , "Baptismal sponsor" , "Mafia boss" .)