Impecunious writer on river vessel (9)
I believe the answer is:
penniless
'impecunious' is the definition.
(synonyms)
'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).
'pen'+'nile'+'ss'='PENNILESS'
(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" .)