Picture one prisoner (4)
I believe the answer is:
icon
'picture' is the definition.
(icon is a kind of picture)
'one prisoner' is the wordplay.
'one' becomes 'i' (Roman numeral).
'prisoner' becomes 'con' (abbreviation for 'convict').
'i'+'con'='ICON'
(Other definitions for icon that I've seen before include "Representative symbol or religious painting" , "Famous and admired person" , "Venerated picture" , "Sacred painting of Greek Orthodox Church" , "Religious image or computer symbol" .)