Penny had a meal on board river vessel (9)
I believe the answer is:
privateer
'vessel' is the definition.
(privateer is a kind of vessel)
'penny had a meal on board river' is the wordplay.
'penny' becomes 'p'.
'had a meal' becomes 'ate'.
'on board' means one lot of letters goes inside another.
'ate' placed inside 'river' is 'rivateer'.
'p'+'rivateer'='PRIVATEER'
(Other definitions for privateer that I've seen before include "Government-commissioned vessel authorised to prey on enemy shipping" , "Drake, for example" , "Pare rivet down for a ship authorised to make war" , "Non-government owned warship" , "Sailor may go on it" .)