Thus American spies left an informal gathering (6)
'an informal gathering' is the definition.
(I've seen this before)
'thus american spies left' is the wordplay.
'thus' becomes 'so' (synonyms).
'american spies' becomes 'CIA' (Central Intelligence Agency).
'left' becomes 'L' (common abbreviation).
(Other definitions for social that I've seen before include "Pertaining to human relations", "Relating to society", "Gregarious; gathering", "Relating to people living in groups", "Informal party".)