Informal social gathering (3-8)
I think it could be either:
get it together
'informal social gathering' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for get-together that I've seen before include "Informal meeting" , "An informal party" , "informal social gathering" , "Assemble" .)
(Other definitions for get it together that I've seen before include "succeed" , "Achieve one's full potential" .)