Retired priest following northern river (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'retired priest following northern' is the wordplay.
'retired' shows that the letters should be reversed in order.
'priest' becomes 'eli' (biblical figure).
'following' is a charade indicator (letters next to each other).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'eli'+'n'='elin'
'elin' in reverse letter order is 'NILE'.
(Other definitions for nile that I've seen before include "River of eastern Africa" , "River at Luxor" , "long-distance runner in Africa" , "River associated with Cleopatra" , "'The longest river, blue and white (4)'" .)