Chat in party's brilliant (6)
I believe the answer is:
confab
'chat' is the definition.
('confab' can be a synonym of 'chat')
'party's brilliant' is the wordplay.
'party' becomes 'con' (abbreviation for Conservatives).
'brilliant' becomes 'fab'.
'con'+'fab'='CONFAB'
'in' is the link.
(Other definitions for confab that I've seen before include "Talk informally" , "Exchange" , "Chat (colloquial)" , "Chat (informal)" , "Informal conversation" .)