Group that’s very large including agents abroad (6)
I believe the answer is:
social
'group' is the definition.
(social group)
'very large including agents abroad' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'large' becomes 'L' (eg in clothes sizes).
'including' is an insertion indicator.
'agents abroad' becomes 'CIA' (US intelligence agency).
'so'+'l'='sol'
'sol' placed around 'cia' is 'SOCIAL'.
'that's' acts as a link.
(Other definitions for social that I've seen before include "Friendly - party" , "Enjoying companionship" , "Gregarious or neighbourly" , "Living in an organised community in Licosa" , "Relating to society" .)