Very good spies start to like being gregarious (6)
I believe the answer is:
social
'gregarious' is the definition.
(associated in meaning)
'very good spies start to like' is the wordplay.
'very good' becomes 'so' (as in the adjectival form 'just so!').
'spies' becomes 'CIA' (Central Intelligence Agency).
'start to' says to take the initial letters.
The initial letter of 'like' is 'l'.
'so'+'cia'+'l'='SOCIAL'
'being' acts as a link.
(Other definitions for social that I've seen before include "Informal gathering" , "Liking companionship" , "Relating to people living in groups" , "Gregarious; a party" , "Of people; gathering" .)