Admiral finally leaves River Plate, manoeuvring armed vessel (9)
I believe the answer is:
privateer
'armed vessel' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for privateer that I've seen before include "Armed ship not owned by a government" , "Armed non-government vessel commissioned for war" , "Non-naval vessel of old, licensed to attack" , "Drake, for example" , "Pare rivet down for a ship authorised to make war" .)