Prisoner with fantastic chatter (6)
I believe the answer is:
confab
'chatter' is the definition.
(I know that chatter can be written as confab)
'prisoner with fantastic' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'with' says to put letters next to each other.
'fantastic' becomes 'fab' (short for fabulous).
'con'+'fab'='CONFAB'
(Other definitions for confab that I've seen before include "natter" , "casual conversation" , "gossip" , "Informal conversation" , "Informal discussion (colloq.)" .)