Social event or illicit relationship (6)
I believe the answer is:
affair
I believe this is a double definition.
'social event' is the first definition.
(affair is a kind of social event)
'illicit relationship' is the second definition.
(I've seen this before)
(Other definitions for affair that I've seen before include "Event; fling" , "Sexual liaison - concern" , "Singular business" , "connected with love" , "Business; romance" .)