Tattler back in group is so gregarious (6)
I believe the answer is:
gossip
'tattler' is the definition.
(thesaurus)
'back in group is so gregarious' is the wordplay.
'back' is a reversal indicator.
'in' indicates the answer is hidden within the clue.
'pissog' is hidden in the letters of 'group is so gregarious'.
'pissog' backwards is 'GOSSIP'.
(Other definitions for gossip that I've seen before include "Schmooze" , "Idle chat, tattle" , "Casual inaccurate talk about others" , "muck spreader" , "Idle talk and rumour" .)