Impecunious writer on river vessel (9)
I believe the answer is:
'impecunious' is the definition.
'writer on river vessel' is the wordplay.
'writer' becomes 'pen' (something which writes).
'on' is a charade indicator (letters next to each other).
'river' becomes 'nile' (Nile is an example).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
(Other definitions for penniless that I've seen before include "Without money, not a cent" , "Poverty-stricken" , "Skint, without a cent" , "Not rich" , "Jack in New York" .)