Formal gathering (8)
I believe the answer is:
function
This clue is a double definition.
'formal' is the first definition.
(thesaurus)
'gathering' is the second definition.
(function is a kind of gathering)
(Other definitions for function that I've seen before include "social gathering" , "Formal event; mathematic expression" , "An official ceremony" , "Formal social event" , "What a thing is used for" .)