This is how spies left informal gathering (6)
I believe the answer is:
social
'informal gathering' is the definition.
(social event)
'this is how spies left' is the wordplay.
'this is how' becomes 'so' (as in 'like so').
'spies' becomes 'CIA' (Central Intelligence Agency).
'left' becomes 'l' (common abbreviation).
'so'+'cia'+'l'='SOCIAL'
(Other definitions for social that I've seen before include "Public servant caring for the underprivileged" , "Outgoing, extroverted" , "Official helping families with problems" , "Sort of security" , "Gregarious or neighbourly" .)