Like this, spies left gathering (6)
I believe the answer is:
social
'gathering' is the definition.
(social gathering is a kind of gathering)
'like this spies left' is the wordplay.
'like this' becomes 'so' ('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 "Of society or its organisation" , "Informal party" , "Needing companionship" , "Sort of security" , "Gregarious; gathering" .)