Be a prisoner or a guiding light (6)
I believe the answer is:
beacon
'a guiding light' is the definition.
(I've seen this in another clue)
'be a prisoner' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'be'+'a'+'con'='BEACON'
'or' is the link.
(Other definitions for beacon that I've seen before include "Warning light on tower or hill" , "A warning light as on headland" , "Shiner" , "Guiding device - signal fire" , "guidance" .)