Prisoner on leave in African river (5)
I believe the answer is:
congo
'african river' is the definition.
(I've seen this in another clue)
'prisoner on leave' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'leave' becomes 'go' (to go is to depart or leave).
'con'+'go'='CONGO'
'in' acts as a link.
(Other definitions for congo that I've seen before include "Second longest river of Africa" , "African river flowing into the South Atlantic" , "Africa country and river" , "Brazzaville its capital" , "African river (aka Zaire)" .)