Do brilliant talk (6)
I believe the answer is:
confab
'talk' is the definition.
(I've seen this before)
'do brilliant' is the wordplay.
'do' becomes 'con'.
'brilliant' becomes 'fab'.
'con'+'fab'='CONFAB'
(Other definitions for confab that I've seen before include "gossip" , "jaw" , "Exchange" , "Chat (colloquial)" , "Chat (informal)" .)