Prisoner having to leave by river (5)
I believe the answer is:
congo
'river' is the definition.
(I know that Congo is a river)
'prisoner having to leave' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'having' is a charade indicator (letters next to each other).
'to leave' becomes 'go' (to go is to depart or leave).
'con'+'go'='CONGO'
'by' acts as a link.
(Other definitions for congo that I've seen before include "Wide river" , "African river (aka Zaire)" , "Region of central Africa" , "Brazzaville its capital" , "Central African country" .)