Crossword Genius

Secret eastern river boat (9)

Ross

I believe the answer is:

privateer

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Telegraph.
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