Surname prisoner keeps saying (8)
I believe the answer is:
cognomen
'surname' is the definition.
('cognomen' can be a synonym of 'surname')
'prisoner keeps saying' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'keeps' is an insertion indicator.
'saying' becomes 'gnome' (gnome is a kind of saying).
'con' placed around 'gnome' is 'COGNOMEN'.
(Other definitions for cognomen that I've seen before include "Family name" , "Nickname -- eg non-com (anag)" , "Epithet" , "Con gnome using kind of nickname" .)