Picture of 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'
'of' is the link.
(Other definitions for icon that I've seen before include "Venerated person (or thing)" , "Devotional image (C, not K)" , "On-screen image" , "Example of symbolism" , "object of admiration" .)