Thus spies left party (6)
I believe the answer is:
social
'party' is the definition.
(I know that social is a type of party)
'thus spies left' is the wordplay.
'thus' becomes 'so' (synonyms).
'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 "Gregarious; gathering" , "Seeking companionship" , "Informal gathering" , "Relating to humans living together in large groups" , "Gregarious or neighbourly" .)