Be a prisoner's guiding light (6)
I believe the answer is:
beacon
'guiding light' is the definition.
(I've seen this before)
'be a prisoner's' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'be'+'a'+'con'='BEACON'
(Other definitions for beacon that I've seen before include "Signal fire - signalling buoy" , "Watch fire" , "Shiner" , "Navigational guide" , "Hilltop signal fire" .)