Secret eastern river boat (9)
I believe the answer is:
privateer
'boat' is the definition.
(privateer is a type of ship)
'secret eastern river' is the wordplay.
'secret' becomes 'private' ('private' can be similar in meaning to 'secret').
'eastern river' becomes 'er' (this could be a standard abbreviation I've not previously seen).
'private'+'er'='PRIVATEER'
Can you help me to learn more?
(Other definitions for privateer that I've seen before include "Vessel commissioned to plunder in wartime" , "Government-sanctioned pirate" , "Warship operating under a letter of marque" , "Sailor may go on it" , "Armed non-government vessel commissioned for war" .)