Social killjoy (5,6)
I believe the answer is:
party pooper
'social killjoy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for party pooper that I've seen before include "Spoilsport" , "Social 1 across" , "One who spoils a social occasion" , "One spoiling others' fun" .)